Organ System

Organ System Definition

An organ system is a group of organs that work together to perform a certain function in an organism’s body. Most animals and plants have organs, which are self-contained groups of tissues such as the heart that work together to perform one function. Humans and other mammals have many organ systems. An example of an organ system is the circulatory system, which includes the heart, arteries, veins, and capillaries. The human body has 11 different organ systems.

This figure depicts the respiratory system.

Examples of Organ Systems

The human organ systems are:

• Integumentary


• Skeletal


• Muscular


• Circulatory


• Respiratory


• Digestive


• Urinary


• Immune


• Nervous


• Endocrine


• Reproductive

The Integumentary System

The integumentary system consists of external organs that protect the body from damage, including the skin, fingernails, and hair. Skin is the largest organ of the human body. It is made up of three layers: the epidermis, dermis, and hypodermis, which contains stored body fat. Nails and hair are both made up of the protein keratin. In other animals, the integumentary system may include feathers, scales, or hooves.

Besides protecting the internal organs from physical damage, the integumentary system has multiple other functions such as protecting against virus invasion, dehydration, sunburns, and changes in temperature, making Vitamin D through sun exposure, and excreting waste through sweating.

The Skeletal System

The skeletal system is made up of all the bones in the human body, i.e., the skeleton. The skeleton forms the supporting structure of the body. It comes from the Greek σκελετός (skeletós), meaning “dried up”, referring to the dry nature of bones. A human infant has 270 bones, some of which fuse together to form the 206 bones in the adult human body. Cartilage is the precursor to bone when an embryo is developing, and it is found in some structures in the human body such as the nose, ears, and joints.

An internal support structure in an animal is called an endoskeleton. Some animals such as insects have hard coverings called exoskeletons on the outside instead of inside the body.

The Muscular System

The muscular system includes the different types of muscles in the body: cardiac, smooth, and skeletal muscles. Cardiac muscles are found only in the heart and contract to pump blood. Smooth muscles are found in organs such as the stomach, intestines, and bladder and move without conscious effort by the organism. Skeletal muscles are attached to bones and work together with bones to move the body.

The Circulatory System

The circulatory system, also known as the cardiovascular system, consists of the heart, veins, arteries, and capillaries. The circulatory system circulates blood throughout the body in order to transport nutrients and oxygen to the cells. The lymphatic system, which includes lymph and lymph nodes, is also part of the circulatory system; lymph transports fats, destroys bacteria, and returns proteins and interstitial fluid from the bloodstream.

Humans and other vertebrates have closed circulatory systems, where the blood is enclosed within blood vessels like veins and arteries. Some animals, such as insects, have open circulatory systems, where blood is pumped into body cavities without the use of vessels.

The Respiratory System

The respiratory system is made up of the organs used for breathing, including the lungs, diaphragm, trachea, bronchi, and bronchioles. In the lungs, oxygen and carbon dioxide are exchanged between the outside air and the blood. Other animals breathe through gills or even through their skin.

The Digestive System

The digestive system digests food and absorbs it into the body. It is made up of the gastrointestinal tract (which includes the esophagus, stomach, liver, and intestines) along with accessory organs of digestion. These include the tongue, liver, pancreas, and gallbladder.

The Urinary System

The urinary system gets rid of wastes from the body in the form of urine. The kidneys, ureters, bladder, and urethra are all part of the urinary system. Sometimes the organs of the urinary system are grouped together with organs that remove wastes such as the skin, lungs, and large intestine, and this is called the excretory system.

The Immune System

The immune system is an organism’s defense system; it protects against disease. Important parts of the immune system include leukocytes (white blood cells), bone marrow, and the thymus. There are many different types of white blood cells, like helper T cells, killer T cells, and B cells. The lymphatic system is also associated with the immune system.

The Nervous System

The nervous system sends and interprets signals from different parts of the body and organizes the body’s actions. The central nervous system includes the brain and spinal cord, while the peripheral nervous system is made up of nerves that allow the central nervous system to connect to the rest of the body.

The Endocrine System

The endocrine system is comprised of all the glands in the body that produce hormones, which are carried via the bloodstream to affect other organs. Some important glands are the pituitary gland, which produces reproductive and many other body-regulating hormones; the thyroid, which has roles in metabolism and protein synthesis; and the adrenal glands, which produce adrenaline and stimulate the fight-or-flight response.

The Reproductive System

The reproductive system includes an organism’s sex organs. In females, some of the sex organs are the vagina, uterus, and ovaries. In males, some sex organs are the penis, testes, prostrate, and vas deferens. All of these organs play a role in sexual reproduction.

Related Biology Terms

• Organ – a self-contained group of tissues that performs a specific function.


• Biological system – a network of entities that are biologically related; this can be on the scale of tissues or organs, but also be used on a larger scale to represent populations of living things.


• Organism – an individual living thing, such as one animal, plant, fungus, or bacterium.

Test Your Knowledge

1. Which organ is NOT a part of the nervous system?
A. Brain
B. Nerves
C. Heart
D. Spinal cord

Answer to Question #1

C is correct. The brain, spinal cord, and nerves make up the nervous system. These organs function together to control the body’s movements and to send and receive signals to and from the other parts of the body. The heart is not a part of the nervous system; it is part of the circulatory system.

2. Which organ system is made up of all the glands that produce hormones?
A. The immune system
B. The muscular system
C. The endocrine system
D. The integumentary system

Answer to Question #2

C is correct. The endocrine system is comprised of the glands that produce hormones, such as the adrenal glands, the pituitary gland, and the thyroid. The immune system is made up of organs that fight disease, the muscular system represents all the muscles in the body, and the integumentary system is made up of external organs that protect the body, like skin, hair, and nails.

3. What is the function of the respiratory system?
A. To digest food and absorb its nutrients into the body
B. To form a supporting structure for the body’s other organs
C. To circulate blood throughout the body and transport oxygen and nutrients to cells
D. To take in oxygen and expel carbon dioxide when an organism breathes

Answer to Question #3

D is correct. The respiratory system is made up of the organs used for breathing (respiration), such as lungs, the trachea, bronchi, and bronchioles. When an organism breathes, the organs of the respiratory system take in oxygen and removes carbon dioxide from the body.

Organ System

Anticodon

Anticodon Definition

Anticodons are sequences of nucleotides that are complementary to codons. They are found in tRNAs, and allow the tRNAs to bring the correct amino acid in line with an mRNA during protein production.

During protein production, amino acids are bound together into a string, much like beads on a necklace. It’s important that the correct amino acids be used in the correct places, because amino acids have different properties. Putting the wrong one in a spot can render a protein useless, or even dangerous to the cell.

This graphic shows a growing protein chain. Towards the bottom left, you can see tRNAs carrying amino acids entering the ribosome complex. If all goes well, only the tRNAs with the correct anticodons will bind successfully to the exposed mRNA, so only the correct amino acids will be added:

tRNAs are responsible for bringing the correct amino acids to be added to the protein, according to the mRNA’s instructions. Their anticodons, which pair-bond with codons on mRNA, allow them to perform this function.

Function of Anticodons

The function of anticodons is to bring together the correct amino acids to create a protein, based on the instructions carried in mRNA.

Each tRNA carries one amino acid, and has one anticodon. When the anticodon successfully pairs up with an mRNA codon, the cellular machinery knows that the correct amino acid is in place to be added to the growing protein.

Anticodons are necessary to complete the process of turning the information stored in DNA into functional proteins that a cell can use to carry out its life functions.

How Anticodons Work

When genetic information is to be turned into a protein, the sequence of events goes like this:

1. Genetic information in the cell’s genome is transcribed into mobile pieces of RNA using base-pairing rules. Each nucleotide has only one other nucleotide which pairs up with it.
   
   By pairing the correct RNA nucleotide with each DNA nucleotide, RNA polymerase creates a strand of RNA that contains all the correct information to make the protein.
   
   This “messenger RNA,” or “mRNA,” then travels to a ribosome, the site of protein production.


2. At the ribosome, the rules of base-pairing are again used to ensure a correct transfer of information. Each three-nucleotide “codon” in the mRNA is matched with an “anticodon” containing the complementary bases.
   
   The “transfer RNAs” or “tRNAs” that string proteins together each have one anticodon that corresponds to one mRNA codon, and one amino acid attached.
   
   When the correct tRNA finds the mRNA, its amino acid is added to the growing protein chain.
   
   Enzymes catalyze the bonding of amino acids together as tRNA anticodons bind to the correct mRNA codon.
   
   When the tRNA’s amino acid has been added to the protein chain, the tRNA leaves to pick up a new amino acid to bring to a new mRNA.
   
   Interestingly, this means that the tRNA anticodon has the RNA version of the same nucleotide sequence of the original gene.
   
   Remember – the gene was transcribed using complementary nucleotides to make RNA, which then had to bond with complementary tRNA codons.

RNA Base Pairing Rules

Each RNA nucleotide can only hydrogen bond to one other nucleotide. It is by bonding the correct nucleotides together that DNA and RNA successfully transfer and use information.

The four bases of RNA are Adenine, Cytosine, Guanine, and Uracil. These bases are often referred to by just their first letter, to make it easier to show sequences of many bases. Base pairing rules for RNA are:

A – U

C – G

G – C

U – A

Put more simply, in RNA, A nucleotides always bond with U nucleotides, and C nucleotides always bond with G nucleotides.

Differences Between RNA and DNA

Of note, in DNA, the “Uracil” base is a slightly different base called “Thymine.” In DNA, A and T pair. RNA Adenine will also pair with DNA’s Thymine, and DNA Adenine will pair with RNA’s Uracil.

The difference between Uracil and Thymine is that Thymine has an extra methyl group, which makes it more stable than Uracil.

It is thought that DNA uses Thymine instead of Uracil because, as the cell’s “master blueprints,” information stored in DNA must remain stable over a long period of time. RNAs are only copies of DNA made for specific purposes, and are used by the cell for only a short period of time before being discarded.

Examples of Anticodons

Let’s look at some examples of DNA base triplets, mRNA codons, and tRNA codons to see if you can fill in the missing information using base pairing rules.

You might find it useful to use a pencil and paper to allow you to transcribe each nucleotide’s complement instead of doing it in your head.

1. mRNA codon: GCU

What is the tRNA anticodon that will bind to this mRNA codon?

Answer to Question #1

CGA. The codon GCU codes for the amino acid alanine, so the tRNA with the corresponding anticodon will be carrying that amino acid.

2. mRNA codon: ACA

What is the corresponding tRNA anticodon?

Answer to Question #2

UCU. The codon CGA codes for the amino acid cysteine, so a tRNA with anticodon UCU will be carrying cysteine.

3. DNA base triplet: CTT

What is the mRNA codon that will be transcribed from this DNA triplet?

Answer to Question #3

GAA. This mRNA codon codes for the amino acid glutamate.

4. Based on the information in the answers to the question above, what is one anticodon for a tRNA that carries glutamate?

Answer to Question #4

CUU. This anticodon that is complementary to an mRNA codon for glutamate.

Related Biology Terms

• Amino Acid – The building blocks of protein. Different amino acids have different properties, which allow cells to build proteins to serve many different functions by stringing the right combinations of amino acids together


• Codon – A three-nucleotide sequence in an mRNA molecule that codes for a particular amino acid. Most amino acids have more than one codon that codes for them, although methionine only has one.


• DNA – The substance used to store the permanent operating instructions of a cell. Information stored in DNA is stable, and can be copied to make new blueprints for daughter cells using nucleotide base pairing rules.

Test Your Knowledge

1. Which of the following is NOT true of anticodons?
A. They are found on tRNAs.
B. They are complementary to codons.
C. They have the RNA equivalent of the same nucleotide sequence as the original DNA instructions for the amino acid.
D. They have the same nucleotide sequence as codons.

Answer to Question #1

D is correct. Anticodons are complementary to codons, NOT the same as them.

2. Which of the following sequences is complementary to: GCUCGU
A. GGAGCA
B. CCACGA
C. CGAGCA
D. CGUGCU

Answer to Question #2

C is correct. It’s helpful to transcribe the sequence letter by letter before answering multiple choice questions like these.

3. Which of the following is something that would NOT be coded for by a codon?
A. Glutamine
B. Glucose
C. Alanine
D. Stop protein production

Answer to Question #3

B is correct. tRNAs do not carry sugars. Sugars may be added to proteins later to form important substances like glycoproteins, but that is done during a later step of protein processing.

Anticodon

Angiosperm

Angiosperm Definition

Angiosperms are a major division of plant life, which make up the majority of all plants on Earth.

Angiosperm plants produce seeds encased in “fruits,” which include the fruits that you eat, but which also includes plants you might not think of as fruits, such as maple seeds, acorns, beans, wheat, rice, and corn.

Angiosperms are also known as “flowering plants” because flowers are a characteristic part of their reproductive structure – though again, you may not always recognize their flowers as the pretty, colorful petaled things you think of when you hear the word.

Angiosperms evolved between 250-200 million years ago. They quickly gained an advantage over the previously dominant plant type – gymnosperms – for two reasons.

Angiosperms’ use of flowers to reproduce made them more reproductively successful. While gymnosperms relied primarily on the wind to achieve sexual reproduction by transferring pollen – which contain the male reproductive cells for plants – into the ovaries of female plants, angiosperms used sweet-smelling, brightly-colored flowers and sugary nectar to attract insets and other animals.

This process of cooperation, whereby animals like bees pollinate flowers in exchange for nectar, made angiosperms more reproductively successful.

Angiosperms also began to encase their seeds in fruits, which both provided extra nourishment and protection for their offspring plants, and created new ways to cooperate with animals. Many angiosperm’s fruits, like their flowers, were designed to attract animals to eat them.

In many cases the seeds would then pass safely through the animals’ digestive tracts, getting carried far from the parent plant in the process. The seeds would eventually be excreted in fecal matter, which, as an added bonus, is often very nutrient-rich for plants. This enabled angiosperms to spread far and wide.

Today angiosperms make up about 80% of all plant species on Earth.

Gymnosperms, which include pines, redwoods, gingko trees, and palm trees, still hold an important place in several ecosystems. But many species of gymnosperms that lived in prehistoric forests are now extinct, having been replaced by angiosperms.

Angiosperm Anatomy

Scientists define angiosperms as plants that have several unique anatomical structures. These include:

• Stamens, which produce the pollen grains that act like sperm for angiosperm plants. Pollen grains contain male genetic information, and can be combined with female genetic information in a plants’ ovaries.
  
  Some angiosperms can fertilize their ovaries with their own pollen, or can reproduce without being fertilized at all. But plants – and organisms in general – that exchange genes through sexual reproduction tend to have more diverse offspring, which means their offspring are more likely to be able to weather disease, predation, and natural disasters.


• Pollen, the angiosperm male reproductive material, which is smaller than the male reproductive materials of gymnosperms.
  
  This means that angiosperm male reproductive cells can reach female eggs faster and with higher success rates than gymnosperm reproductive cells.


• Flowers, which are structures that contain the male and female reproductive parts of an angiosperm – and which are often designed to attract insects and other animals that can perform cross-pollination between different plants.


• Carpels, which enclose the ovaries that are are found inside or just behind the plant’s flower. Ovaries can receive pollen grains and begin producing seeds and fruit more rapidly than gymnosperms can produce their own seeds.

If you watch a plant’s development carefully, you can see the base of the flower swell and develop into fruit after pollination. This is the process of the carpel, which surrounds the plant’s ovary, growing into a fruit around the developing seeds.

In many fruits, the woody “spot” on the bottom opposite the planet’s stem shows where the flower was once attached, before the carpel grew into a fruit.

Examples of Angiosperms

Examples #1: Fruits

Fruit trees are perhaps the most obvious illustration of the angiosperm’s life cycle.

Fruit trees often shows flowers, such as apple, cherry, and orange blossoms, before they bear fruit. These flowers are pollinated by bees or other animals, allowing fruit trees to exchange genetic material and keep their population diverse.

Once the flowers have served their purpose of attracting pollinators, they lose their petals, and the carpels at the base of the flower begin to swell. These carpels continue to grow until the fruit has reached full-size, and may change color to better attract animals that might want to eat it.

When a tree’s fruit is eaten by birds or ground-dwelling animals, its seeds get a free ride to wherever that animal is going – and free fertilizer, in the form of the manure it will be excreted with.

Examples #2: Grains

It might seem strange to think of grasses flowering plants, but they are indeed a member of the flowering plant family.

Grasses have moved away from their evolutionary origin of attracting animal pollinators with big, colorful flowers and fruit. Because grasses like wheat and rice often grow in large numbers very close together, they can rely on the wind to pollinate them, and to spread their seeds through the environment.

The versions of rice, corn, and wheat that humans eat has seeds that could be described as “freakishly large,” because we have been selectively breeding our domesticated crops to have the largest possible seeds for thousands of years.

As such, these domesticated plants often don’t produce well without humans, because their seeds are too large to be carried by the wind. However, as long as humans are around, we will plant lots and lots of them to feed ourselves!

In the wild, the seeds of grasses are much smaller and are easily spread by wind.

Examples #3: Vegetables

The vegetables that come to our dinner plates have also been selectively bred by humans for many generations to make them as big, and tasty, as possible. As such, it may surprise you to hear that broccoli, kale, and lettuce are all flowering plants!

Broccoli, kale, and lettuce that are to be eaten are typically harvested before they flower, since flowers are not considered delicious by most humans. The tight, green buds that make up broccoli plants are just that – tiny flower buds!

Farmers and gardners will typically allow some of their green vegetables to flower and produce seeds, so that they can plant them for next year’s harvest. But green vegetables meant to be eaten are usually picked before their flowers show.

Examples #4: Flowers

When it comes to flowers that were bred to be big and bright, your question might be “where on Earth does the fruit come in?”

The truth is that not all fruits look like the big, colorful, sweet fruits we think of when we hear the term. In fact, a “fruit” is any protective layer around a seed, and many plants’ “fruits” may just look like swollen seed pods.

Many flowers, including roses, lilies, and daffodils, produce swollen green seed pods where the flowers used to be, after their petals have dropped. If you walk through a daffodil garden after the flowers have lost their petals, you may see the stems “nodding” as they become heavy with the weight of the developing fruit.

If you leave the seed pods on the stems long enough, they will eventually take a dried-out appearance. If you can shake the seed pod and hear dried seeds rattling around inside, that means that the seed’s maturation process has finished, and you can harvest the seeds to grow more daffodils next year.

The much-touted “rosehips” which are sometimes used in food or medicinal preparations are actually the fruit of the rose plant!

Related Biology Terms

• Plant – A living organism that turns energy from sunlight into fuel for cells, using the process of photosynthesis. Plants are the base of most ecosystems’ energy pyramids, as animals eat plants to absorb some of the energy they derive from the sun.


• Seed – A plant’s unit of reproduction, which includes the genetic material and any necessary nutrients to start the development of a new plant.


• Symbiosis – A relationship between two organisms in which both benefit. The cooperation between angiosperms and animals could be seen as an example of symbiosis.

Test Your Knowledge

1. Which of the following is NOT a difference between gymnosperms and angiosperms?
A. Angiosperms reproduce sexually, gymnosperms do not.
B. Angiosperms have smaller pollen, making pollination more efficient.
C. Angiosperms use flowers to attract pollinating animals.
D. Gymnosperms rely on the wind to carry their pollen.

Answer to Question #1

A is correct. Both angiosperms and gymnosperms use sexual reproduction to ensure population diversity. Angiosperms just do it a little more efficiently, using techniques described in points B-D.

2. Which of the following is NOT a part of an angiosperm’s flower?
A. Stamen
B. Cone
C. Carpel
D. Petal

Answer to Question #2

B is correct. Cones are actually a feature of gymnosperm reproduction. These non-tasty coverings protect their seeds from being eaten.

3. Which of the following edible plants is not an angiosperm?
A. Almond
B. Wheat
C. Pine nuts
D. Lettuce

Answer to Question #3

C is correct. Pine nuts are harvested from pine cones. They are the seeds of pine trees, which are gymnosperms. Unlike the nuts of angiosperm plants, pine nuts are not intended by the pine trees to be eaten. But humans found a way to do it!

Angiosperm

Analogous Structures

Analogous Structures Definition

Analogous structures are similar structures that evolved independently in two living organisms to serve the same purpose.

The term “analogous structures” comes from the root word “analogy,” which is a device in the English language where two different things on a basis of their similarities.

Analogous structures are examples of convergent evolution, where two organisms separately have to solve the same evolutionary problem – such as staying hidden, flying, swimming, or conserving water – in similar ways. The result is similar body structures that developed independently.

In the case of analogous structures, the structures are not the same, and were not inherited from the same ancestor. But they look similar and serve a similar purpose.

For example, the wings of an insect, bird, and bat would all be analogous structures: they all evolved to allow flight, but they did not evolve at the same time, since insects, birds, and mammals all evolved the ability to fly at different times.

Examples of Analogous Structures

Examples #1: Wings Through The Ages

As mentioned above, many creatures have independently developed wings. All wings were evolved in order to solve the same problem: how to fly through the air. But they have evolved on several different occasions throughout history.

Insects were the first organisms to evolve structures which could push air down in order to propel their bodies through the air. Insects probably evolved flight by using parts of their protective exoskeletons to propel themselves through the air.

Millions of years later, reptiles learned to do the same thing- pterosaurs evolved a skin membrane, stretched between their finger and ankle bones, which was capable of propelling them through the air.

Millions of years later still, dinosaurs separately evolved flight – using the feathers they had developed to keep warm in order to push them into the sky. In the process, these small, feathered dinosaurs evolved into birds.

Mammals solved the problem of flight yet again about 100 million years after birds first appeared, with bats using a similar solution to that of the pterosaurs: skin membranes stretched between long finger bones.

In this way, we have at least four different types of wings in the fossil record which are analogous: they serve the same purpose, but were not inherited from the same ancestor.

Examples #2: The “Duck-Billed” Platypus

When the first specimen of a platypus was sent to a British museum by an Australian explorer, they tried to pry it apart to prove it was a fake! British scientists were sure that someone had simply stuck a duck’s bill onto the body of a beaver-like animal.

However, the truth was much more interesting: platypi had evolved almost exactly the same structure evolved by ducks to solve the problem of gathering food such as fish and aquatic plants from water.

Ducks and platypi could not possibly be related – platypi are mammals, and they evolved long after birds and mammals went their separate ways on the evolutionary path. Yet both evolved very similar solutions when they moved from land back into the water!

Examples #3: Cacti and Water Conservation

Some members of the plant genres Euphorbia and Astrophytum look extremely similar.

Both have round, ball-shaped bodies divided into eight equal wedges; both have hard, pointy thorns sticking out in a row along the middle of each wedge, protecting them from animals who might try to eat them. To the untrained eye, they may be mistaken for members of the same species.

This is particularly remarkable because these two genii are only distantly related, and they live in two completely different parts of the world.

Astrophytum evolved in North America, and all members of its genus are cacti that live in the southwestern deserts.

Euphorbia, on the other hand, is a plant genus that includes poinsettias – as well as certain cacti found in the deserts of Africa.

Both the African and North American cacti conserve water by minimizing their surface area – resulting in a round, ball shape – developing a thick, waxy skin, and placing prickly deterrents on its skin at its most vulnerable places to discourage animals from trying to eat it for its moisture.

The result is two plants which look nearly identical – but which have very different ancestry!

Difference Between Analogous and Homologous Structures

The difference between homologous and analogous structures can be thought of in terms of ancestry and function:

• Analogous structures have different ancestry, but the same function.

These can be thought of in terms of the literary device of “analogy,” where two different things are compared based on their similarities.

• Homologous structures have the same ancestry, but may no longer serve the same function.

For example, the bones that make up human fingers were inherited from an ancestor that’s shared by all mammals. Bats, dogs, and whales also have these bones, but bats use them to spread their wings, dogs walk on them, and whales do not use them for anything since they are encased inside their fins.

These structures are therefore homologous – there is a clear relationship and similarities between them, even though they are not used for the same purpose.

The existence of homologous structures is strong evidence for the theory of evolution, since there is no reason why a whale should have the same bones in its fin that a bat has in its wings, unless they both evolved from a common ancestor.

These can be thought of in terms of the literary device of “homonyms,” where two words sound the same, but have different meanings.

Identifying Analogous Structures

Scientists usually identify analogous structures by looking at the known relatives of the two species being studied.

If a line of common inheritance can be found – such as humans and monkeys both having fingers, when we have a fossil record showing that humans and monkeys shared a common ancestor, who also had fingers – the structures are not considered analogous.

But if no common ancestor which shares these features is found – such as in the case of bats and insects, whose shared ancestor did not fly at all – the structures would be considered analogous.

Related Biology Terms

• Convergent evolution – When two species independently evolve similar solutions to an evolutionary problem.


• Evolution – The process by which organisms change over time, due to natural selection and survival of the fittest.

Test Your Knowledge

1. Which of the following is NOT an example of convergent evolution?
A. Cacti in North America and Africa that have the same body structures.
B. Birds and bats that are both able to fly.
C. Whales and bats that both have metacarpal “finger” bones.
D. Platypi and ducks that both have duck bills.

Answer to Question #1

C is correct. Whales and bats did not evolve metacarpals to serve the same purpose – they inherited them from a common ancestor.

2. Which of the following is true of analogous structures?
A. They have the same function, and the same ancestry.
B. They have the same function, but different ancestry.
C. They have the same ancestry, but may have different functions.
D. They have different ancestry, and different functions.

Answer to Question #2

B is correct. Analogous structures are structures which perform the same function, but have different ancestry.

3. We know that organisms must have evolved structures independently if they do not have a common ancestor.
A. True
B. False

Answer to Question #3

True & False. This is a trick question. It is true that organisms must have evolved structures independently if they don’t have a common ancestor who has the structure in question.

Birds and bats could not have inherited wings from a shared ancestor, for example, because the earliest mammals from whom bats descended did not have wings.

However, all organisms living on Earth have a common ancestor somewhere in their past. It’s just a question of whether that common ancestor is the source for their shared trait, or whether it evolved in both species independently after their family trees diverged.

Analogous Structures

Giant Anaconda

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A living, reptilian nightmare, the anaconda dwells in the dim jungles and sluggish rivers of the Amazon Basin. Anacondas are the apex predators of apex predators; they gorge on jaguars and crocodiles. Yet, according to legend, there might be a serpent even mightier than the anaconda hidden in the Amazon.

What is a Giant Anaconda?

The Giant Anaconda is an exaggerated version of the already monstrous anacondas living in South America. The legendary serpents are often reported as being two or three times the size of the largest anaconda to be scientifically documented.

Characteristics

Physical Description

Giant Anacondas are quite similar to regular anacondas; in fact, people who report seeing Giant Anacondas usually claim that the snake was a terrifically large specimen of the known species.

Pound for pound, the largest snake species in the world is the green anaconda. The largest green anaconda on record is 29 feet (8.8 meters) long and weighs 550 lbs. (227 kilograms) — a true gargantuan measuring a full 14 feet (4 meters) more than the average length. Still, Giant Anacondas dwarf even the largest green anaconda on record. Giants top the scales at 165 feet (50 meters) long.

Like regular anacondas, Giant Anacondas have dark green-grey skin and egg-shaped black spots. They may also have yellowish spots near their underbelly and orange bars that run from their eyes to their nightmarish jaws. Both their jaws and their skin can expand, allowing them to swallow meals of a ghastly size.

A terrible stench is sometimes associated with Giant Anacondas. Percy Fawcett, a famous explorer who reported encountering a 62-foot anaconda in Bolivia, described the stench as

“a penetrating foetid odor […] probably its breath.”

Other explorers reported a similarly unbearable smell, usually caused by the anaconda’s breath.

The Giant Anacondas of native legend are even more terrifying. They may be pitch black, have horns, or have large, fiery eyes.

Behavior

Most Giant Anaconda reports come out of South America, where vast swamps and marshes allow these monsters to lie hidden under the lilies. Giants have also been reported lurking in the dense jungle vegetation of the Amazon Rainforest and in some African rainforests.

Giant Anacondas are often spotted right after they’ve eaten. Crocodiles, jaguars, humans, and boats are said to be common prey, and the etymology of the word anaconda (which means “having killed an elephant” in Tamil) suggests that Giant Anacondas might be capable of tackling even larger prey.

Giant Anacondas are constrictors, just like their smaller relatives. They first attack their prey, pinning it between the inward-curving teeth of their massive jaws, then wrap coil upon mighty coil around their prey and squeeze until the heart stops beating. By some accounts, they might have breath so terrible or a gaze so fearsome that it can paralyze their prey before they attack.

According to native South American legends, Giant Anacondas may be shape-shifters that spend part of their lives in human form. Giant Anacondas are often depicted as guardians as well, sometimes barring the way to a hidden treasure, sometimes acting as protectors of nature itself.

Related Creatures

Although Giant Anacondas probably don’t slither the Earth today, the ancestors of today’s anacondas did reach giant proportions. Titanoboa is the largest snake species the world has seen. This prehistoric, Columbian monster is estimated to have reached lengths of 42 feet (13 meters) or more. Gigantophis, who held the title of the world’s largest snake before remains of Titanoboa were unearthed, lived in the Middle East and was capable of growing to 36 feet (11 meters) or more. Both snakes were constrictors, like the Giant Anaconda.

Beyond the fossil record, South American folklore is populated with numerous anaconda-esque monsters.

Yacumama is a Goddess who takes the form of a giant sea-serpent, believed by the indigenous people of Peru, Ecuador, and Argentina to inhabit the mouth of the Amazon River. Yacumama is also referred to as “the mother of the water” and is believed to have given life to all the animals in the sea.

Boitata is a river-serpent with fiery powers. According to Brazilian folklore, the Boitata was a cave-dwelling anaconda who developed an appetite for eating the eyes of his prey. After eating hundreds of eyes, his own eyes became filled with a furious fire, which he can now use to set fire to forests and fields. Fortunately, the serpent has a protective spirit and uses his power over fire to protect the forest and villages from errant flames, rather than setting the world ablaze himself.

Boiuna is a colossal, shape-shifting snake. He spends most of his time in the depths of lakes and slow-moving rivers, but when he does emerge from the water to travel through the jungle, he is so large that he fells trees and carves new rivers along his way.

M’boi is a serpent-god who lives in rivers. In a fit of rage, he broke the Earth and created the magnificent Iguazu falls, which can be seen at the border between Brazil and Argentina.

Cultural Representation

Origin

Legends about a great, water-dwelling serpent stretch far back into the murky depths of Amazonian culture, but “Giant Anaconda” lore wasn’t popularized until European colonists brought the influence of the more scientific western world to South America. Colonists may not have believed in the mystical powers that indigenous people assigned to their god-like snakes, but they were very ready to believe, and contribute to, reports of mammoth snakes hiding in the river.

Famous Reports

One of the most interesting aspects of Giant Anaconda mythology is its ability to capture the attention of both cryptozoologists (scientists who specialize in studying mythological or fringe species) and zoologists (who usually stand firmly in the skeptic’s camp about legendary creatures). Indeed, some of the most famous Giant Anaconda reports have come from mainstream zoologists.

In 1906, a famous British explorer named Percy Fawcett claimed that he shot a 62.3-foot (19 meter) anaconda while prowling along the Brazil-Bolivia border. The claim, however, did not stand up among his fellow naturalists back home. He was widely ridiculed for his Giant Anaconda story.

On the heels of Fawcett’s flop, Vincent Roth, a famous arachnologist who spent many years surveying the wildlife in Guyana, made a more plausible claim of having shot an anaconda that measured 33.8 feet (10.3 meters). Unfortunately, he was unable to produce evidence to support his claim.

Historian Mike Dash also published photos of Giant Anacondas, although his photos didn’t include objects which could be used as guides for scale.

Art and Literature

Although the Giant Anaconda is incredibly elusive in the real world, he is no stranger to the spotlight of fictional worlds.

One of Uruguay’s most beloved writers, Horacio Quiroga, wove Giant Anacondas into many of his jungle tales. Eventually, in 1921, he even named one of his books Anaconda, in honor of the powerful beast.

The Giant Anaconda is also a darling of Hollywood’s horror movie industry. The Anaconda movie series has grossed well over $200,000,000 and has seen crossovers with another popular horror movie franchise, Lake Placid.

Explanations of the Myth

There’s no question that the Giant Anaconda is inspired by real anacondas, which can grow to sizes that make them worthy of horror movie stardom even without embellishment.

So why does the idea of a bigger and badder anaconda persist?

Easy: people truly believe they’ve seen Giant Anacondas.

Estimating an anaconda’s size in the wild is extremely difficult. The animals are rarely stretched straight, ready to be measured. Their bulk is usually curved, coiled, or even submerged under water—so it’s easy for a person to imagine a few extra meters tacked onto these truly monstrous snakes.

Of course, some brave (and destructive) souls turn their anaconda sightings into anaconda killings, but even after an anaconda is killed, documenting its size in a credible way is difficult. Photographs can be edited, and snake skins can be stretched, so neither one is considered to be reliable evidence of a Giant Anaconda in the scientific community.

Myths thrive on uncertainty, and in the case of the Giant Anaconda, uncertainty has allowed for myth to grow to an incredible size.

Giant Anaconda

Organ

Organ Definition

An organ is a self-contained group of tissues that performs a specific function in the body. The heart, liver, and stomach are examples of organs in humans. The word organ comes from the Latin organum, which means “instrument”. This in turn comes from the Greek word ὄργανον (órganon), which refers to a musical instrument or “organ of the body”. Organs are found in most animals and plants.

Examples of Organs

All animals except for less specialized ones like those in the phylum Porifera (sea sponges) have specialized tissues grouped into organs. The human body has 78 different organs. The largest organ is the skin, while the smallest organ is the pineal gland, which produces the hormone melatonin. Some organs are necessary for survival; these are called vital organs. Humans have five vital organs that they cannot live without: the brain, heart, liver, kidneys, and lungs. (A person can live with one kidney instead of two, or one lung, but they must have one working kidney or lung to survive.)

Other organs perform important functions, but a person can live without them. For example, if a person has stomach cancer and undergoes a total gastrectomy, their entire stomach is removed, and their esophagus is surgically connected to their small intestine. They may suffer some hardships and may have to alter their diet by eating smaller meals and taking nutritional supplements, but they can still digest food in their intestines just fine, and can therefore live without a stomach. Other examples of non-essential organs include the bladder, spleen, and gallbladder.

Some organs are vestigial, meaning that they perform little or no function in the body. Evolutionarily, over time, they have become unnecessary. The appendix, a small tube connected to the large intestine that can sometimes become inflamed and require removal, has been widely thought to be vestigial. It no longer has a significant role in digestion. However, it does still play a role in immune functioning and maintaining good gut bacteria.

Although we may think of plants as being simpler than animals, plants also have organs. This includes reproductive organs such as stamens and pistils (contained in flowers), roots, stems, and leaves. Each of these organs performs specialized tasks such as reproduction, absorbing nutrients from the soil, and performing photosynthesis.

Cells, Tissues, Organs, Organ Systems

There are four different levels of organization in multicellular organisms, and organs make up one of these levels. From simplest to complex, an organism is made up of cells, tissues, organs, and organ systems. Cells make up the most basic level of organization; the cell is the building block of a living organism. This is followed by tissues. Tissues are groups of cells that work together and have a similar structure and function. The four types of tissues in the human body are muscle, epithelial, connective, and nervous tissue. Organs, as stated before, are groups of tissues that work together to perform a certain function. Organ systems represent the highest level of an organism’s bodily organization. They are made up of groups of organs that work together in order to carry out a certain function. For example, the digestive system includes organs such as the esophagus, stomach, small intestine, and large intestine, and all of these organs play a role in the digestion of food.

Types of Organs

The body’s organs are grouped into organ systems based on the functions they perform. Humans have 11 different organ systems. Here are all of the organ systems with some examples of organs found in each system:

• Integumentary (skin, hair, nails)


• Skeletal (bones)


• Muscular (smooth, cardiac, and skeletal muscles)


• Circulatory (heart, arteries, veins)


• Respiratory (lungs, diaphragm, larynx)


• Digestive (stomach, intestines, liver)


• Urinary (kidneys, ureters, bladder)


• Immune (lymph nodes, bone marrow, thymus)


• Nervous (brain, spinal cord, nerves)


• Endocrine (pituitary gland, thyroid, adrenals)


• Reproductive (penis, vagina, prostate, uterus)

This image depicts parts of the circulatory system, including the heart, arteries, and veins:

Related Biology Terms

• Tissue – a specialized group of cells that look similar and perform a specific function; groups of similar tissues make up organs.


• Organ system – a group of organs that works together to carry out a certain function. Humans have 11 organ systems.


• Vital organ – an organ that has evolutionarily lost its original function over time, and presently has little or no function in an organism. The appendix has been widely believed to be vestigial throughout history, although it does have a role in immune functioning.

Test Your Knowledge

1. Organs are self-contained groups of what?
A. Organ systems
B. Tissues
C. Cells
D. Organisms

Answer to Question #1

B is correct. While organs are technically made up of cells (as is everything else in an organism), organs themselves are defined as groups of specialized tissues that work together to carry out a function in the body. In turn, groups of organs that work together make up organ systems. An entire organism is made up of different organ systems.

2. Which is NOT a vital organ in humans?
A. Bladder
B. Brain
C. Heart
D. Liver

Answer to Question #2

A is correct. The bladder performs an important role in collecting urine that is to be excreted from the body, but it is not necessary for a person to have a bladder in order to survive. If the bladder is removed during a process called a cystectomy, which is sometimes performed on a patient with bladder cancer, a person can still survive. Instead, urine will be collected outside the body in a urostomy bag. The five vital organs that are needed for survival in humans are the brain, heart, liver, kidneys, and lungs.

3. Which living thing does not have organs?
A. Shark
B. Apple tree
C. Horse
D. Sea sponge

Answer to Question #3

D is correct. Sea sponges are multicellular and classified as animals, but they are very simple; they do not have any organs or even true tissues. Instead, they are made up of groups of cells. Other animals and plants have specialized tissues and organs.

Organ

Nucleotide

Nucleotide Definition

Nucleotides are organic molecules that are the building blocks of DNA and RNA. They also have functions related to cell signaling, metabolism, and enzyme reactions. Nucleotides are made up of three parts: a phosphate group, a 5-carbon sugar, and a nitrogenous base. The four nitrogenous bases in DNA are adenine, cytosine, guanine, and thymine. RNA contains uracil instead of thymine. Nucleotides make up the DNA and RNA of all living things.

Parts of the Nucleotide

The figure below shows the three parts of the nucleotide. From left to right, each nucleotide is made up of a phosphate group, a 5-carbon sugar, and a nitrogenous base (the one shown here is adenine).


In DNA, the 5-carbon sugar is deoxyribose, while in RNA, the 5-carbon sugar is ribose. This gives DNA and RNA their names; the full name of DNA is deoxyribonucleic acid, and RNA is ribonucleic acid.

DNA and RNA contain all the genetic information necessary for cells to function. In DNA and RNA, many nucleotides are bonded together to form long strands in a structure called a double helix. The phosphate group and the 5-carbon sugar compose the backbone of the double helix, while the nitrogenous bases are located in the middle and are bonded to each other.

This figure shows the chemical structure of DNA. Note how the phosphates and sugars are bonded to form a backbone, and the nitrogenous bases are also bonded to each other in the middle of the double helix:

Types of Nitrogenous Bases in Nucleotides

The five types of nitrogenous bases that are found in nucleotides are called adenine, cytosine, guanine, thymine, and uracil. They are often abbreviated to A, C, G, T, and U.

Adenine

Adenine is a purine, which is one of two families of nitrogenous bases. Purines have a double-ringed structure. In DNA, adenine bonds with thymine. In RNA, adenine bonds with uracil.

Cytosine

The other family of nitrogenous bases is pyrimidines. Cytosine is a pyrimidine; it has only one ring in its structure. Cytosine bonds with guanine in both DNA and RNA.

Guanine

Like adenine, guanine is a purine; it has a double ring. It bonds with cytosine in DNA and RNA.

Thymine

Like cytosine, thymine is a pyrimidine and has one ring. It bonds with adenine in DNA. Thymine is not found in RNA.

Uracil

Uracil is also a pyrimidine. It bonds with adenine in RNA; it is not found in DNA.

More on Nitrogenous Bases

These paired nitrogenous bases make up the middle of the double helix structure. A purine always binds with a pyrimidine, but more specifically, each base bonds with its complementary base: A with T (or U, in RNA), C with G, and vice versa. When the nitrogenous bases of two nucleotides are bonded, they are referred to as base pairs. The bases are connected via hydrogen bonds. Hydrogen bonds can easily detach so that the DNA can “unzip” during the replication process.

Sometimes when DNA is replicated, the wrong nitrogenous base is inserted into the copy of the DNA. There are mechanisms in place to correct these errors, but some go through unnoticed with the result that the DNA does not have a proper base pairing at the location. These are called point mutations and can affect a gene’s functioning. Most point mutations are harmless, but if they occur in sperm or egg cells, they can be passed on to offspring. Sickle cell anemia is an example of a disorder that is caused by a single point mutation in the gene that creates hemoglobin, which is the part of red blood cells that carries oxygen to the rest of the body.

Functions of Nucleotides

Besides being the basic unit of genetic material for all living things, nucleotides have other functions as well. Nucleotides are found in other molecules such as adenosine triphosphate (ATP), which is the main energy molecule of the cell. They are also found in coenzymes like NAD and NADP, which come from ADP; these molecules are used in many chemical reactions that play roles in metabolism. Another molecule that contains nucleotides is cyclic AMP (cAMP), a messenger molecule that is important in many processes including the regulation of metabolism and transporting chemical signals to cells. Nucleotides not only make up the building blocks of life, but also form many different molecules that function to make life possible.

Related Biology Terms

• Deoxyribonucleic acid (DNA) – a molecule that contains all the genetic instructions that allow an organism to function.


• Ribonucleic acid (RNA) – a molecule similar to DNA that plays a role in many activities such as protein synthesis, gene expression, and aiding chemical reactions.


• Nitrogenous base – a nitrogen-containing molecule that is one of the components of a nucleotide. The nitrogenous bases in DNA and RNA are adenine, cytosine, guanine, thymine, and uracil.


• Purine – one of the two families of nitrogenous base pair molecules. Purines have a double-ring structure; the other family is the pyrimidines, which have a single ring.

Test Your Knowledge of Nucleotides

1. Which is NOT one of the parts of a nucleotide?
A. 5-carbon sugar
B. Phosphate group
C. Phospholipid
D. Nitrogenous base

Answer to Question #1

C is correct. The three components of a nucleotide are a 5-carbon sugar, a phosphate group, and a nitrogenous base. Nucleotides do not contain phospholipids; those are molecules that make up the cell membrane and nuclear envelope.

2. Which is a correct pairing?
A. Adenine–Guanine
B. Cytosine–Guanine
C. Thymine–Uracil
D. Uracil–Cytosine

Answer to Question #2

B is correct. Cytosine always pairs with guanine in both DNA and RNA, and vice versa. Cytosine is a pyrimidine, and this allows it to pair with guanine, which is a purine.

3. Which nitrogenous base is not found in DNA?
A. Uracil
B. Thymine
C. Adenine
D. Guanine

Answer to Question #3

A is correct. Uracil is not found in DNA. It is only found in RNA, where it replaces thymine (which is not found in RNA). Uracil binds with adenine in RNA.

Nucleotide

Ratatoskr

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Scurrying up and down the Norse tree of life is Ratatoskr, a squirrel with a taste for mischief. Ratatoskr may have big responsibilities as a messenger for the gods, but he doesn’t accept his duties meekly. This crafty squirrel puts his own spin on his role.

What is Ratatoskr?

Ratatoskr (Ratr) is a squirrel who carries messages along Yggdrasil, the tree of life. The most regular subscribers to his messaging service are the wise eagle who sits at the top of Yggdrasil, and the hungry dragon, Nidhoggr, who lies coiled among the tree’s roots. Ratatoskr relishes the chance to ferry an insult between these two mighty beasts, and by doing so, he is continually stirring the animosity between them.

Characteristics

Physical Description

Ratatoskr is described as a red squirrel. In ancient artwork, he is depicted with extremely long ears, but this could be an artifact of the art style of the time, rather than a meaningful statement about Ratr’s physique. Texts that describe Ratr don’t mention him having any features that set him apart from your typical bright-eyed and bushy-tailed red squirrel.

Personality

Ratatoskr is regarded as a troublemaker. He enjoys fueling spiteful relationships, and he may sometimes add his own embellishments to the messages sent between the eagle and Nidhoggr the dragon.

Some scholars believe that Ratatoskr may have higher ambitions than just inflaming the fraught relationship between the eagle and Nidhoggr. In some interpretations, Ratatoskr is intent on destroying the tree of life. Because he lacks the strength to do much damage to the tree by himself, Ratatoskr manipulates the eagle and the dragon into attacking the tree, which stands between them and the opportunity to fight each other. Ratr tells Nidhoggr of a particularly vicious comment the eagle made about him, and the dragon gnaws at the roots of the tree, hoping to cause it to fall and crush the eagle. Then, Ratr returns to the eagle with the news that Nidhoggr is gnawing at the tree, in an attempt to do him harm. The eagle begins to pluck branches from the tree and rain them down on Nidhoggr. With his well-placed accusations, Ratatoskr succeeds at doing great damage to the tree of life. Occasionally, the squirrel might even chip into the effort with his reputed “gnawing teeth.”

Related Characters

Ratatoskr is sometimes linked to Rati, a magical drill which the god Odin used on a quest to obtain magical mead, made from the blood of the wisest man who ever lived, out of a fortress inside a mountain. Rati was used to drill a hole in the mountain. Odin used this hole to enter the mountain in the shape of a serpent, swallowed all the mead of wisdom, and turned into an eagle before exiting the mountain again. Still in the form of an eagle, Odin ascended to Asgard (located in the top branches of Yggdrasil) to share the mead with the other gods.

The numerous parallels between Rati, in the tale of the stolen mead, and Ratatoskr, in descriptions of the tree of life, have caused many scholars to believe that Ratatoskr symbolizes Rati. Both Ratatoskr and Rati are frequently described as “gnawing.” Both allow communion between an underworld and a higher place of wisdom, and both draw a connection between a snake-like creature and an eagle. Some scholars even believe that the most direct translation of “Ratatoskr” is “Rati’s tooth.”

Ratatoskr might also symbolize the god Heimdall (also called Guillintanni or Vindler), who is renowned for his keen eyesight and hearing, and for his golden teeth. Heimdall guards the bridge between Asgard, the land of the Gods, and Midgard, the world of humanity. He is charged with sounding a warning if the bridge is ever under attack.

If Ratatoskr is, indeed, meant to represent Heimdall, then the squirrel probably has a worse reputation than he deserves. It may be his duty, rather than his desire to cause trouble, that compels Ratatoskr to carry bad news between the top and bottom of Yggdrasil. He is responsible for warning the two worlds of any threat brewing on the other side.

Outside of Norse mythology, a multitude of other messengers and mischief-makers take the form of a squirrel. The one who most resembles Ratr is Meeko, a red squirrel who wreaks havoc throughout Native American legends. Meeko has highly destructive tendencies, but because he is so small, he usually uses the anger of other creatures to carry out his schemes.

Cultural Representation

Literature

Ratatoskr appears in all three of the major literary reference points for Norse mythology: The Codex Regius, The Poetic Edda, and The Edda. These volumes span the thirteenth and fourteenth centuries and contain collections of anonymous poems and legends that pre-date the Christianization of Norse culture.

Over time, Ratatoskr has come under the scrutiny of allegorical writers and even psychoanalysts.

In his book Teutonic Mythology, Jacob Grimm, one of the famous fairytale writing Brothers Grimm, puzzled over the deeper meaning of Ratr’s role in Norse mythology. He concluded that Ratatoskr helped maintain balance in the tree of life. Yes, he caused tension and destruction, but that tension and destruction helped keep the tree straight and allowed it to put out new growth.

In an essay titled “Ratatosk: The Role of the Perverted Intellect,” Lilla Vesy-wagner, a psychoanalyst, linked Ratatoskr’s intelligence with his taste for conflict. She argued that intelligence, like Ratr has, often causes conflict between the demands of the id and the superego, which might be symbolized by the wise eagle and the hungry dragon.

Visual Arts

Little artwork remains from the heyday of pre-Christian Norse culture, so most depictions of Ratatoskr have been touched by the influences of Christianity. Over time, the symbols of the Norse tree of life and the Christian cross became intertwined, as can be seen in the famous Ruthwell and Bewcastle crosses. These large stone crosses are rich with carvings that harken back to “pagan” imagery. Squirrels can be seen climbing up and down the crosses—but they look much tamer than the mischievous Ratr of old.

In the art and entertainment of today, Ratr makes a return to his old self. He is a popular melee character in the video game Smite, and he appears in Marvel Comics as a supervillain let loose by Loki to spread chaotic ideas and rumors on earth.

Explanation of the Myth

It’s not difficult to guess why a red squirrel was selected to play the role of Yggdrasil’s fussy messenger—nor why the squirrel has played a similar role in the legends of cultures across the world. Nimble, loud, and somewhat aggressive, squirrels just fit the bill. It’s easy to imagine that a squirrel who barks at you for walking near the hole where he has hidden his acorns, then rushes up tree and vanishes with one flourish of his fluffy tail, is going to fill the ear of some god with an abusive account of your behavior. The squirrels who fussed at Norsemen from the canopies of Europe’s old forests probably inspired Ratatoskr.

Ratatoskr