The Lone Ranger
08-04-2008, 11:17 AM
An Introduction to Zoology
Chapter Four: Animal Body Plans and Phylogenies:
Body Plans:
An organism’s body plan is the set of distinctive morphological and developmental traits that is characteristic of the taxon to which it belongs. It’s important to remember that a particular body plan may have evolved more than once in life’s history though, so it can be a mistake to think that just because two different species have similar body plans that they’re necessarily closely-related.
Extant animal species display a relatively small number of body plans. Once a particular body plan evolved, it seems to have remained remarkably stable. For instance, comparisons between species that are separated by hundreds of millions of years of evolutionary history reveal that the molecular control of gastrulation has not changed in over 500 million years.
Given the remarkable stability of animals’ body plans, they have often been used to classify organisms. For example, all animals with radial symmetry are often assumed to make up a clade, while animals with bilateral symmetry are often assumed to make up a different clade. Nowadays, we have the ability to directly compare organisms’ genetic and biochemical makeups in order to determine their evolutionary relationships. What these molecular techniques sometimes reveal is that just because organisms share similar body plans doesn’t always mean that they’re related.
[b]Body Symmetry:
Almost all animals have bodies that are distinctly symmetrical. If an animal’s body is symmetrical, you can draw an imaginary line through the center of the animal’s body that divides it into right and left halves that are mirror images of each other.
Because it’s so easy to see what kind of body symmetry an animal has, body symmetry is often used as a means of classifying animals. A few animals, such as sponges for instance, have bodies that display no symmetry at all. The vast majority of animals, however, are either radially symmetrical or bilaterally symmetrical. The fossil record indicates that these two basic types of body symmetry have been around for at least 550 million years.
http://www.freethought-forum.com/forum/gallery/files/5/0/sponge_original.jpg
Sponges (Phylum Porifera), unlike the vast majority of animals, do not have symmetrical bodies.
[b]Radial Symmetry:
An animal shows radial symmetry if its body can be divided into mirror-image halves in more than one way. Such an animal has a top side and a bottom side, but no front and no back. Neither do radially-symmetrical animals have left and right sides.
Animals that have radial symmetry include sea anemones, sea stars (“starfishes”), and “jellyfishes” (properly known as jellies). Most of these animals move very slowly or not at all. An animal that is more or less permanently attached to some surface and cannot move on its own is said to be sessile. Most animals that are sessile are radially symmetrical.
Radial symmetry may provide some advantages, but it seems to come with a number of disadvantages, too. A radially-symmetrical animal has no front and no back, no right and no left; such animals typically have their sense organs scattered around the body surface instead of concentrated in one region. This gives a radially-symmetrical animal the ability to sense danger (or prey) from any direction. On the other end, since radially-symmetrical animals are almost always either sessile or very slow-moving, it seems that radial symmetry is not compatible with quick and coordinated movement.
http://www.freethought-forum.com/forum/gallery/files/5/0/radial.jpg
A hydra (phylum Cnidaria) has a radially-symmetrical
body. Note that there are many different ways that
its body could be divided into mirror images.
[b]Bilateral Symmetry:
An animal shows bilateral symmetry if its body can be divided into mirror-image halves in only one way. Such an animal has a distinctive anterior (front) end and posterior (back) end, as well as a right side and a left side.
Most animals are bilaterally symmetrical, and the majority of bilaterally-symmetrical animals show cephalization (from the Greek “kephale,” meaning “head”). “Cephalization” refers to the concentration of the sense organs at the anterior end of the body to form a head, and the concentration of nervous tissue in the same region to form a brain.
While it’s true that bilaterally-symmetrical animals don’t have the advantage of being able to easily sense danger (or prey) from any direction, bilateral symmetry and cephalization seem to provide a number of advantages. For one thing, having the sense organs and brain close-together allows for quick reactions and coordinated movement. Probably not coincidentally, animals with bilateral symmetry tend to be much faster and more coordinated in their movements than are radially-symmetrical animals.
http://www.freethought-forum.com/forum/gallery/files/5/0/bilateral.jpg
A crayfish (Phylum Arthropoda) has a bilaterally-symmetrical body,
like most animals. It also clearly displays cephalization – note the
concentration of the sense organs at the anterior end.
[b]Body Tissues:
Another basic way in which animals’ body plans differ is in how their tissues are organized. You no-doubt recall that a tissue consists of body cells of the same type that together perform a common function. Sponges (Phylum Porifera) and a few other animals lack true tissues, but all other animals have body tissues that develop from the embryonic tissues that are produced during gastrulation.
As you recall, during early development, the blastula folds inward to produce a gastrula. In the process, two layers of embryonic tissues are formed, the inner endoderm and the outer ectoderm. These tissues are known as the germ layers, because all body tissues are ultimately derived from the embryonic tissues. (“Germ,” as in “germinate,” means to sprout or grow.) The ectoderm forms the outer covering of the animal, and in most animals, the central nervous system. The endoderm forms most of the tissues associated with the digestive system.
Animals whose bodies develop from only these two germ layers are said to be diploblastic. These include the jellies, sea anemones, corals and other members of the Phylum Cnidaria. The other phylum of diploblastic animals is the Phylum Ctenophora; these animals are commonly referred to as “comb jellies.”
The cnidarians and ctenophorans are collectively referred to as the Radiata, since almost all of them have radially-symmetrical bodies.
http://www.freethought-forum.com/forum/gallery/files/5/0/jelly.jpg
A jelly (Phylum Cnidaria) is diploblastic – its body develops from only
two embryonic germ tissue layers. Note the lack of complex internal
organs, which are formed by mesodermal tissue in other animals.
http://www.freethought-forum.com/forum/gallery/files/5/0/comb_jelly.jpg
A “comb jelly” (Phylum Ctenophora). Like cnidarians,
ctenophorans are diploblastic and lack complex internal organs.
In most animals, a third embryonic germ layer forms between the endoderm and the ectoderm. This is the mesoderm, and animals whose bodies develop from three layers of embryonic tissue are said to be triploblastic.
The triploblastic animals are collectively referred to as the Bilateria, because almost all of them are bilaterally-symmetrical. In these animals, mesoderm forms muscles and complex internal organs, so the Bilateria typically have much more complex bodies than do the Radiata.
http://www.freethought-forum.com/forum/gallery/files/5/0/germ_layers.jpg
A cross-section through the bodies of a typical vertebrate (Phylum Chordata) and a
typical insect (Phylum Arthropoda). Each body structure is formed from one
of the three embryonic germ layers. Ectoderm (blue) forms skin and the
central nervous system. Mesoderm (red) forms muscles, blood, and most of
the internal organs. Endoderm (yellow) forms most of the tissues associated
with the digestive system.
[b]Body Cavities:
Most triploblastic animals have internal fluid-filled or air-filled body cavities. Such a body cavity is known as a coelom (pronounced see-lum).
A “true” coelom is completely surrounded by mesodermal tissue, and can thus be subdivided into compartments. Animals with a true coelom are known as coelomates.
Some animals possess a “false” coelom called a pseudocoelom. These animals are known as pseudocoelomates. A pseudocoelom is a body cavity that lies between mesodermal and endodermal tissue and is, therefore, not completely surrounded by mesodermal tissue. Because it is not completely surrounded by mesodermal tissue, a pseudocoelom cannot be subdivided into functionally-separate compartments in the way that a true coelom can.
Finally, there are a few triploblastic animals that completely lack any sort of body cavity. These animals are known as acoelomates.
http://www.freethought-forum.com/forum/gallery/files/5/0/coelom.jpg
Acoelomates, pseudocoelomates, and coelomates. An acoelomate
such as a flatworm (Phylum Platyhelminthes) lacks any sort of internal
fluid-filled cavity. A pseudocoelomate such as a roundworm (Phylum Nematoda)
has an internal body cavity that lies between mesodermal and endodermal
tissues. A coelomate such as an earthworm (Phylum Annelida) has an
internal body cavity that is entirely surrounded by mesodermal tissue and
can be subdivided.
A body cavity has many different functions. For one thing, the fluid inside cushions the organs suspended within it, helping to prevent internal injuries. The cavity allows internal organs to grow and move independently of the outer body wall. For example, were it not for your coelom, your body surface would warp noticeably every time your heart beat or your intestines contracted.
Perhaps most importantly, however, a fluid-filled cavity acts as a hydrostatic skeleton for soft-bodied animals. An animal’s body is made mostly of water, and water is incompressible. This means that animals with internal, water-filled cavities have the capacity to resist outside pressure and the pull of gravity, and so can maintain a constant body shape. It’s not coincidental that acoelomate animals have flat bodies – their bodies are flattened by a combination of the pull of gravity and of the pressure exerted by the surrounding air or water. Coelomates and pseudocoelomates don’t have this problem.
Because a coelom or pseudocoelom acts as a hydrostatic skeleton and allows its possessor to resist external pressure, coelomates and pseudocoelomates can burrow, something that few acoelomates can do.
A hydrostatic skeleton doesn’t just help in resisting external pressure and the pull of gravity; it can also be used to redirect the force of muscle contractions. This gives most pseudocoelomates and coelomates more control over their movements than a typical acoelomate can manage. Because the coelom or psedocoelom acts as a hydrostatic skeleton and allows more effective force to be generated by muscle contractions than an acoelomate can manage, most coelomates and pseudocoelomates can swim. Few acoelomates can manage to generate sufficient muscle force to swim.
In the coelomates, the body cavity is almost always subdivided. Because the different compartments can redirect muscle-contraction forces independently, coelomates typically have much finer control over their movements than do pseudocoelomates, much less acoelomates. Another advantage of dividing the body cavity is that each body compartment is semi-isolated. Because body compartments can be isolated, if one section of the body is injured or becomes infected, this need not affect other regions of the body.
http://www.freethought-forum.com/forum/gallery/files/5/0/roundworm_original.jpg
A roundworm (Phylum Nematoda) is a pseudocoelomate. If anything penetrates the outer body tissues and
into the animal’s pseudocoelom, the entire pseudocoelom can drain, killing the animal. Similarly, if disease-
causing organisms get into the pseudocoelom, they can quickly spread through it to the worm’s entire body.
http://www.freethought-forum.com/forum/gallery/files/5/0/earthwormanatomy.jpg
An earthworm (Phylum Annelida) is a coelomate. Note how its body cavity is
subdivided by mesodermal tissue. If the body wall is penetrated, only one
body segment will be affected, so the worm will likely survive. Similarly, if
diseases-causing organisms manage to invade one body segment, they will
find it difficult to move into other body segments. Subcompartmentation
of the body cavity also gives the earthworm a far greater degree of control
over its movements than a pseudocoelomate possesses, since each
body segment can be manipulated independently.
[break=Protostome and Deuterostome Development]
[b]Protostome and Deuterostome Development:
Most triploblastic animals have one of two different developmental modes – protostome development or deuterostome development. These developmental modes can be distinguished by differences in how cleavage occurs, how the coelom forms, and by the ultimate fate of the blastopore.
Cleavage, you recall, is the rapid cellular division that occurs early in development. During cleavage, cells divide so rapidly that little or no growth occurs between cellular divisions. The first cleavage produces two cells, of course. The second cleavage produces four cells. The third division results in eight cells. And so forth. Most protostomes (“proto” = “first” and “stome” = “mouth”) have spiral cleavage.
In spiral cleavage, the third cellular division (as well as subsequent divisions) occurs at an oblique angle to the original body axis. This produces an 8-cell morula in which the four cells in one plane do not lie directly over the four cells in the other plane. Another characteristic of spiral cleavage is that the third cellular division produces cells of distinctly different sizes.
http://www.freethought-forum.com/forum/gallery/files/5/0/spiral_cleavage.jpg
Spiral cleavage: Note how the third cellular division occurs at an
oblique angle to the morula’s polar axis, as do subsequent cellular divisions.
The third cellular division also produces cells of distinctly-different sizes.
[break]
Another characteristic of protostome development is that cleavage is usually determinate. What determinate cleavage means is that the ultimate fate of each cell is determined very early in development. A snail, for instance, is a protostome and has determinate cleavage. If you remove one of the four cells that was formed after the second cleavage in a snail, the embryo will be inviable because many of the necessary body organs will not form.
[break]
In protostomes the coelom typically forms through schizocoelous development. In schizocoelous development, the mesoderm of the gastrula forms as cells from both the endoderm and the ectoderm migrate inward, forming a solid middle layer of cells. The mesoderm then splits down the middle and the two halves separate, leaving a fluid-filled space between them. This fluid-filled space is the embryonic coelom.
http://www.freethought-forum.com/forum/gallery/files/5/0/schizocoelous_original.jpg
Schizocoelous development: The mesoderm forms as cells from the ectoderm
and the endoderm migrate inward, forming a solid layer of cells. The mesoderm
then splits down the middle and separates. The cavity that results,
which is entirely surrounded by mesodermal tissue, is the embryonic coelom.
[break]
You’ll recall that the blastopore, the opening into the archenteron of the gastrula, ultimately forms one of the two openings of the digestive tube. In protostomes, the blastopore forms the mouth. Later in development, a second opening forms at the other end of the digestive tube and becomes the anus.
[break=Deuterostomes]
Most deuterostomes (“deutero” = “second” and “stome” = “mouth”) have radial cleavage. In radial cleavage, cellular divisions occur either in line with the body axis or at right angles to it. This means that, in the 8-cell morula, the four cells in one plane lie directly over the four cells in the second plane. Another characteristic of radial cleavage is that the cellular divisions typically produce cells of the same size.
http://www.freethought-forum.com/forum/gallery/files/5/0/radial_cleavage.jpg
Radial cleavage: Note how the cellular divisions occur either in line with
the body axis or at right angles to it. This produces cells that lie directly
atop each other. Another characteristic of radial cleavage is that
cellular divisions typically result in cells of roughly equal size.
[break]
In deuterostomes, cleavage is usually indeterminate. In indeterminate cleavage, the ultimate fate of cells is not determined until relatively late in development. A human, for instance, is a deuterostome with indeterminate development. If you separate one of the four cells that was formed after the second cleavage from the others, both the separated cell and the three-cell grouping will continue to divide normally, ultimately producing two fully-formed and healthy embryos.
In a deuterostome, if the morula splits in two during early development, identical twins will result. This does not happen in protostomes.
[break]
In deuterostomes, the coelom typically forms through enterocoelous development. In enterocoelous development, the mesoderm of the gastrula forms as two outgrowths of the endoderm. As they grow larger, the two mesodermal pouches grow toward each other. Eventually, they meet and merge. The space between the mesodermal layers forms the embryonic coelom.
http://www.freethought-forum.com/forum/gallery/files/5/0/enterocoelous_original.jpg
Enterocoelous development: The mesoderm forms as two outgrowths of
endodermal tissue. The mesodermal pouches grow toward each other as
they enlarge. Eventually, the two mesodermal pouches meet and fuse.
The space between the mesodermal layers forms the embryonic coelom.
[break]
In deuterostomes, the blastopore forms the anus. Later in development, a second opening forms at the other end of the digestive tube and becomes the mouth.
http://www.freethought-forum.com/forum/gallery/files/5/0/development_524412.jpg
A summary of the developmental differences between protostomes and deuterostomes.
[break=New Views of Animal Phylogeny]
[b]New Views of Animal Phylogeny:
In the past, animals were generally classified according to their body plans. For instance, it was assumed that all animals with a pseudocoelom were probably closely-related. But organisms with similar body plans may not resemble each other because of common ancestry; it may be a result of convergent evolution. So it’s dangerous to assume that just because two different species have similar body plans that they must be close relatives.
Nowadays, we can directly compare the biomolecules (principally proteins) that make up organisms’ bodies, and we can directly compare organisms’ DNA sequences. These molecular techniques allow us to reconstruct animals’ phylogenies with much more certainty than was possible in the past. In some cases, these techniques have shown that animals which appear to be close relatives because of similar body plans are, in fact, only distantly related to each other.
Remember that, when constructing a classification scheme, biologists are hoping to classify organisms into clades. That is, we’re hoping to construct taxa that accurately show evolutionary histories and relationships – phylogenies. As more molecular data are gathered, our understanding of the relationships between animal taxa will continue to be refined, which means that classifications sometimes change in the light of new data.
This means that you shouldn’t think of the phylogenies found in textbooks as set in stone. As new information comes to light, there will inevitably be changes in how organisms are classified. Even so, on a large scale, we can be fairly certain that our classifications truly represent animals’ relationships. For instance, there are about three dozen currently-recognized animal phyla, and that is not likely to change very much. On the other hand, on a smaller scale, there are plenty of animal groups whose phylogenies are still the subject of investigation and of lively debate within the scientific community.
http://www.freethought-forum.com/forum/gallery/files/5/0/traditional.jpg
A “traditional” animal phylogeny, based on comparison of body plans.
(Many minor phyla have been omitted, for clarity.)
The “traditional” way of constructing animal phylogenies was through comparison of body plans. According to this classification technique, there are two main groups of animals, the Parazoa and the Eumetazoa. The parazoans are the sponges (Phylum Porifera) and similar animals. Their defining characteristic is that they lack true tissues. All other animals have true tissues and are traditionally classified as eumetazoans.
In traditional phylogenies, the Eumetazoa are further divided into the Radiata and the Bilateria. The radiates are the jellies, sea anemones and corals (Phylum Cnidaria), the comb jellies (Phylum Ctenophora) and similar animals. Their defining characteristics are that they’re diploblastic and have radial symmetry. The Bilateria are triploblastic and have bilateral symmetry.
Traditional phylogenies subdivide the Bilateria into the Acoelomates, the Pseudocoelomates, and the Coelomates. The Acoelomates include the flatworms (Phylum Platyhelminthes) and similar animals. Their defining characteristic, of course, is that each of them lacks an internal, mesoderm-lined body cavity. The Pseudocoelomates include the roundworms (Phylum Nematoda), rotifers (Phylum Rotifera) and similar animals. They have body cavities, but they aren’t entirely lined by mesodermal tissue and are not subdivided. The Coelomates include animals as diverse as insects (Phylum Arthropoda), sea stars (Phylum Echinodermata), and humans (Phylum Chordata). Their defining characteristic is the possession of an internal body cavity that is entirely lined by mesodermal tissue and can be subdivided.
Traditionally, the Coelomate animals are further subdivided into the Protostomia and the Deuterostomia. Most animals are protosomes; the two major deuterostome phyla are the echinoderms and the chordates.
Some animals, such as the Lophophorates, are notoriously difficult to classify using traditional techniques. Molecular techniques allow us to classify them with far greater certainty. They also show that some of the traditional taxa do not accurately reflect animals’ phylogenies.
[break=Molecular Phylogenies]
http://www.freethought-forum.com/forum/gallery/files/5/0/animal_phylogeny.jpg
A more recent animal phylogeny, based on molecular data.
Molecular data shows that some of the “traditional” taxa are true clades, while others are only grades and are thus in need of revision. The molecular data also strongly suggest that the choanoflagellate protists were the ancestors of all members of the Kingdom Animalia.
According to the molecular data, the Parazoa and the Eumetazoa are true clades. Evidently, animals split into these two groups very early in their history.
The Radiata and the Bilateria are also true clades, according to the molecular data. This is another split that occurred very early in animal history.
Things get more complicated after this point, however, as the molecular data show that the Acoelomates, Pseudocoelomates and Coelomates are not true clades. As such, though the terms are sometimes used to describe animals, they’re no longer used to classify them, since they don’t accurately indicate phylogenies.
The molecular data indicate that the division of the Bilateria into the Protostomia and the Deuterostomia is justified and that these are true clades. Again, the separation between protostomes and deuterostomes occurred very early in animal history, when the most complicated living animals would have resembled worms. (That’s why that episode of Star Trek: The Next Generation in which Lieutenant Barclay “devolved” into a spider-like creature is so funny. Spiders are protostomes; humans are deuterostomes. Therefore, humans most-definitely are [b]not descended from anything that ever looked even [b]remotely like a spider!)
The molecular data show that the Protostomia are divided into two major groups, the Lophotrochozoa and the Ecdysozoa. We’ll discuss each of these groups in later chapters.
Finally, the molecular data show that, as had been suspected from morphological and developmental evidence, the two major phyla of the Deuterostomia – the Echinodermata and the Chordata – are indeed closely-related. Perhaps it’s strange to think of a sea star as a fairly close relative of yours, but that is indeed the case.
[b]Points of Agreement:
Though animal phylogenies are continually being revised as more data become available, there are at least five major points that are considered settled beyond any reasonable doubt. As such, these conclusions are unlikely to change in the light of new data.
The first point of agreement is that the animals are indeed a monophyletic group, and that they’re descended from colonial flagellates, apparently the choanoflagellates. In other words, all living animals are descended from a single common ancestor. Animals, therefore, form a true clade that is known as the Metazoa (Kingdom Animalia).
The second point of agreement is that sponges (Phylum Porifera) are basal animals. That is, the ancestors of the sponges split off from the main line of animal evolution very early.
The third point of agreement is that the Eumetazoa (“true animals”) are a real clade. The Eumetazoa, you recall, are the animals that possess true tissues. According to both traditional classification schemes and the molecular data, all eumetazoans – even animals as radically different as jellies and humans – are descended from a common eumetazoan ancestor.
The fourth point of agreement is that the Bilateria (which includes the great majority of animals) and the Radiata (the phyla Cnidaria and Ctenophora) are true clades. Evidently, all of the triploblastic, bilaterally-symmetrical animals are descended from common ancestry. (Some of the Bilateria are not bilaterally-symmetrical as adults, but all are bilaterally-symmetrical as embryos.) Similarly, the diploblastic, radially-symmetrical animals are descended from common ancestry.
The final point of agreement is that, within the Bilateria, the Protostomia and the Deuterostomia represent true clades. Traditional and molecular phylogenies sometimes disagree on which animals are protostomes and which are deuterostomes, but the molecular data clearly indicate that the Protostomia and the Deuterostomia are real clades.
[break]
By now, we have a basic understanding of how animals develop and of how they’re classified. We also have a basic understanding animal diversity and of the major stages in animal evolution. In the next chapter, we will return to the subject of animal reproduction, to explore it in a little more detail.
Chapter Four: Animal Body Plans and Phylogenies:
Body Plans:
An organism’s body plan is the set of distinctive morphological and developmental traits that is characteristic of the taxon to which it belongs. It’s important to remember that a particular body plan may have evolved more than once in life’s history though, so it can be a mistake to think that just because two different species have similar body plans that they’re necessarily closely-related.
Extant animal species display a relatively small number of body plans. Once a particular body plan evolved, it seems to have remained remarkably stable. For instance, comparisons between species that are separated by hundreds of millions of years of evolutionary history reveal that the molecular control of gastrulation has not changed in over 500 million years.
Given the remarkable stability of animals’ body plans, they have often been used to classify organisms. For example, all animals with radial symmetry are often assumed to make up a clade, while animals with bilateral symmetry are often assumed to make up a different clade. Nowadays, we have the ability to directly compare organisms’ genetic and biochemical makeups in order to determine their evolutionary relationships. What these molecular techniques sometimes reveal is that just because organisms share similar body plans doesn’t always mean that they’re related.
[b]Body Symmetry:
Almost all animals have bodies that are distinctly symmetrical. If an animal’s body is symmetrical, you can draw an imaginary line through the center of the animal’s body that divides it into right and left halves that are mirror images of each other.
Because it’s so easy to see what kind of body symmetry an animal has, body symmetry is often used as a means of classifying animals. A few animals, such as sponges for instance, have bodies that display no symmetry at all. The vast majority of animals, however, are either radially symmetrical or bilaterally symmetrical. The fossil record indicates that these two basic types of body symmetry have been around for at least 550 million years.
http://www.freethought-forum.com/forum/gallery/files/5/0/sponge_original.jpg
Sponges (Phylum Porifera), unlike the vast majority of animals, do not have symmetrical bodies.
[b]Radial Symmetry:
An animal shows radial symmetry if its body can be divided into mirror-image halves in more than one way. Such an animal has a top side and a bottom side, but no front and no back. Neither do radially-symmetrical animals have left and right sides.
Animals that have radial symmetry include sea anemones, sea stars (“starfishes”), and “jellyfishes” (properly known as jellies). Most of these animals move very slowly or not at all. An animal that is more or less permanently attached to some surface and cannot move on its own is said to be sessile. Most animals that are sessile are radially symmetrical.
Radial symmetry may provide some advantages, but it seems to come with a number of disadvantages, too. A radially-symmetrical animal has no front and no back, no right and no left; such animals typically have their sense organs scattered around the body surface instead of concentrated in one region. This gives a radially-symmetrical animal the ability to sense danger (or prey) from any direction. On the other end, since radially-symmetrical animals are almost always either sessile or very slow-moving, it seems that radial symmetry is not compatible with quick and coordinated movement.
http://www.freethought-forum.com/forum/gallery/files/5/0/radial.jpg
A hydra (phylum Cnidaria) has a radially-symmetrical
body. Note that there are many different ways that
its body could be divided into mirror images.
[b]Bilateral Symmetry:
An animal shows bilateral symmetry if its body can be divided into mirror-image halves in only one way. Such an animal has a distinctive anterior (front) end and posterior (back) end, as well as a right side and a left side.
Most animals are bilaterally symmetrical, and the majority of bilaterally-symmetrical animals show cephalization (from the Greek “kephale,” meaning “head”). “Cephalization” refers to the concentration of the sense organs at the anterior end of the body to form a head, and the concentration of nervous tissue in the same region to form a brain.
While it’s true that bilaterally-symmetrical animals don’t have the advantage of being able to easily sense danger (or prey) from any direction, bilateral symmetry and cephalization seem to provide a number of advantages. For one thing, having the sense organs and brain close-together allows for quick reactions and coordinated movement. Probably not coincidentally, animals with bilateral symmetry tend to be much faster and more coordinated in their movements than are radially-symmetrical animals.
http://www.freethought-forum.com/forum/gallery/files/5/0/bilateral.jpg
A crayfish (Phylum Arthropoda) has a bilaterally-symmetrical body,
like most animals. It also clearly displays cephalization – note the
concentration of the sense organs at the anterior end.
[b]Body Tissues:
Another basic way in which animals’ body plans differ is in how their tissues are organized. You no-doubt recall that a tissue consists of body cells of the same type that together perform a common function. Sponges (Phylum Porifera) and a few other animals lack true tissues, but all other animals have body tissues that develop from the embryonic tissues that are produced during gastrulation.
As you recall, during early development, the blastula folds inward to produce a gastrula. In the process, two layers of embryonic tissues are formed, the inner endoderm and the outer ectoderm. These tissues are known as the germ layers, because all body tissues are ultimately derived from the embryonic tissues. (“Germ,” as in “germinate,” means to sprout or grow.) The ectoderm forms the outer covering of the animal, and in most animals, the central nervous system. The endoderm forms most of the tissues associated with the digestive system.
Animals whose bodies develop from only these two germ layers are said to be diploblastic. These include the jellies, sea anemones, corals and other members of the Phylum Cnidaria. The other phylum of diploblastic animals is the Phylum Ctenophora; these animals are commonly referred to as “comb jellies.”
The cnidarians and ctenophorans are collectively referred to as the Radiata, since almost all of them have radially-symmetrical bodies.
http://www.freethought-forum.com/forum/gallery/files/5/0/jelly.jpg
A jelly (Phylum Cnidaria) is diploblastic – its body develops from only
two embryonic germ tissue layers. Note the lack of complex internal
organs, which are formed by mesodermal tissue in other animals.
http://www.freethought-forum.com/forum/gallery/files/5/0/comb_jelly.jpg
A “comb jelly” (Phylum Ctenophora). Like cnidarians,
ctenophorans are diploblastic and lack complex internal organs.
In most animals, a third embryonic germ layer forms between the endoderm and the ectoderm. This is the mesoderm, and animals whose bodies develop from three layers of embryonic tissue are said to be triploblastic.
The triploblastic animals are collectively referred to as the Bilateria, because almost all of them are bilaterally-symmetrical. In these animals, mesoderm forms muscles and complex internal organs, so the Bilateria typically have much more complex bodies than do the Radiata.
http://www.freethought-forum.com/forum/gallery/files/5/0/germ_layers.jpg
A cross-section through the bodies of a typical vertebrate (Phylum Chordata) and a
typical insect (Phylum Arthropoda). Each body structure is formed from one
of the three embryonic germ layers. Ectoderm (blue) forms skin and the
central nervous system. Mesoderm (red) forms muscles, blood, and most of
the internal organs. Endoderm (yellow) forms most of the tissues associated
with the digestive system.
[b]Body Cavities:
Most triploblastic animals have internal fluid-filled or air-filled body cavities. Such a body cavity is known as a coelom (pronounced see-lum).
A “true” coelom is completely surrounded by mesodermal tissue, and can thus be subdivided into compartments. Animals with a true coelom are known as coelomates.
Some animals possess a “false” coelom called a pseudocoelom. These animals are known as pseudocoelomates. A pseudocoelom is a body cavity that lies between mesodermal and endodermal tissue and is, therefore, not completely surrounded by mesodermal tissue. Because it is not completely surrounded by mesodermal tissue, a pseudocoelom cannot be subdivided into functionally-separate compartments in the way that a true coelom can.
Finally, there are a few triploblastic animals that completely lack any sort of body cavity. These animals are known as acoelomates.
http://www.freethought-forum.com/forum/gallery/files/5/0/coelom.jpg
Acoelomates, pseudocoelomates, and coelomates. An acoelomate
such as a flatworm (Phylum Platyhelminthes) lacks any sort of internal
fluid-filled cavity. A pseudocoelomate such as a roundworm (Phylum Nematoda)
has an internal body cavity that lies between mesodermal and endodermal
tissues. A coelomate such as an earthworm (Phylum Annelida) has an
internal body cavity that is entirely surrounded by mesodermal tissue and
can be subdivided.
A body cavity has many different functions. For one thing, the fluid inside cushions the organs suspended within it, helping to prevent internal injuries. The cavity allows internal organs to grow and move independently of the outer body wall. For example, were it not for your coelom, your body surface would warp noticeably every time your heart beat or your intestines contracted.
Perhaps most importantly, however, a fluid-filled cavity acts as a hydrostatic skeleton for soft-bodied animals. An animal’s body is made mostly of water, and water is incompressible. This means that animals with internal, water-filled cavities have the capacity to resist outside pressure and the pull of gravity, and so can maintain a constant body shape. It’s not coincidental that acoelomate animals have flat bodies – their bodies are flattened by a combination of the pull of gravity and of the pressure exerted by the surrounding air or water. Coelomates and pseudocoelomates don’t have this problem.
Because a coelom or pseudocoelom acts as a hydrostatic skeleton and allows its possessor to resist external pressure, coelomates and pseudocoelomates can burrow, something that few acoelomates can do.
A hydrostatic skeleton doesn’t just help in resisting external pressure and the pull of gravity; it can also be used to redirect the force of muscle contractions. This gives most pseudocoelomates and coelomates more control over their movements than a typical acoelomate can manage. Because the coelom or psedocoelom acts as a hydrostatic skeleton and allows more effective force to be generated by muscle contractions than an acoelomate can manage, most coelomates and pseudocoelomates can swim. Few acoelomates can manage to generate sufficient muscle force to swim.
In the coelomates, the body cavity is almost always subdivided. Because the different compartments can redirect muscle-contraction forces independently, coelomates typically have much finer control over their movements than do pseudocoelomates, much less acoelomates. Another advantage of dividing the body cavity is that each body compartment is semi-isolated. Because body compartments can be isolated, if one section of the body is injured or becomes infected, this need not affect other regions of the body.
http://www.freethought-forum.com/forum/gallery/files/5/0/roundworm_original.jpg
A roundworm (Phylum Nematoda) is a pseudocoelomate. If anything penetrates the outer body tissues and
into the animal’s pseudocoelom, the entire pseudocoelom can drain, killing the animal. Similarly, if disease-
causing organisms get into the pseudocoelom, they can quickly spread through it to the worm’s entire body.
http://www.freethought-forum.com/forum/gallery/files/5/0/earthwormanatomy.jpg
An earthworm (Phylum Annelida) is a coelomate. Note how its body cavity is
subdivided by mesodermal tissue. If the body wall is penetrated, only one
body segment will be affected, so the worm will likely survive. Similarly, if
diseases-causing organisms manage to invade one body segment, they will
find it difficult to move into other body segments. Subcompartmentation
of the body cavity also gives the earthworm a far greater degree of control
over its movements than a pseudocoelomate possesses, since each
body segment can be manipulated independently.
[break=Protostome and Deuterostome Development]
[b]Protostome and Deuterostome Development:
Most triploblastic animals have one of two different developmental modes – protostome development or deuterostome development. These developmental modes can be distinguished by differences in how cleavage occurs, how the coelom forms, and by the ultimate fate of the blastopore.
Cleavage, you recall, is the rapid cellular division that occurs early in development. During cleavage, cells divide so rapidly that little or no growth occurs between cellular divisions. The first cleavage produces two cells, of course. The second cleavage produces four cells. The third division results in eight cells. And so forth. Most protostomes (“proto” = “first” and “stome” = “mouth”) have spiral cleavage.
In spiral cleavage, the third cellular division (as well as subsequent divisions) occurs at an oblique angle to the original body axis. This produces an 8-cell morula in which the four cells in one plane do not lie directly over the four cells in the other plane. Another characteristic of spiral cleavage is that the third cellular division produces cells of distinctly different sizes.
http://www.freethought-forum.com/forum/gallery/files/5/0/spiral_cleavage.jpg
Spiral cleavage: Note how the third cellular division occurs at an
oblique angle to the morula’s polar axis, as do subsequent cellular divisions.
The third cellular division also produces cells of distinctly-different sizes.
[break]
Another characteristic of protostome development is that cleavage is usually determinate. What determinate cleavage means is that the ultimate fate of each cell is determined very early in development. A snail, for instance, is a protostome and has determinate cleavage. If you remove one of the four cells that was formed after the second cleavage in a snail, the embryo will be inviable because many of the necessary body organs will not form.
[break]
In protostomes the coelom typically forms through schizocoelous development. In schizocoelous development, the mesoderm of the gastrula forms as cells from both the endoderm and the ectoderm migrate inward, forming a solid middle layer of cells. The mesoderm then splits down the middle and the two halves separate, leaving a fluid-filled space between them. This fluid-filled space is the embryonic coelom.
http://www.freethought-forum.com/forum/gallery/files/5/0/schizocoelous_original.jpg
Schizocoelous development: The mesoderm forms as cells from the ectoderm
and the endoderm migrate inward, forming a solid layer of cells. The mesoderm
then splits down the middle and separates. The cavity that results,
which is entirely surrounded by mesodermal tissue, is the embryonic coelom.
[break]
You’ll recall that the blastopore, the opening into the archenteron of the gastrula, ultimately forms one of the two openings of the digestive tube. In protostomes, the blastopore forms the mouth. Later in development, a second opening forms at the other end of the digestive tube and becomes the anus.
[break=Deuterostomes]
Most deuterostomes (“deutero” = “second” and “stome” = “mouth”) have radial cleavage. In radial cleavage, cellular divisions occur either in line with the body axis or at right angles to it. This means that, in the 8-cell morula, the four cells in one plane lie directly over the four cells in the second plane. Another characteristic of radial cleavage is that the cellular divisions typically produce cells of the same size.
http://www.freethought-forum.com/forum/gallery/files/5/0/radial_cleavage.jpg
Radial cleavage: Note how the cellular divisions occur either in line with
the body axis or at right angles to it. This produces cells that lie directly
atop each other. Another characteristic of radial cleavage is that
cellular divisions typically result in cells of roughly equal size.
[break]
In deuterostomes, cleavage is usually indeterminate. In indeterminate cleavage, the ultimate fate of cells is not determined until relatively late in development. A human, for instance, is a deuterostome with indeterminate development. If you separate one of the four cells that was formed after the second cleavage from the others, both the separated cell and the three-cell grouping will continue to divide normally, ultimately producing two fully-formed and healthy embryos.
In a deuterostome, if the morula splits in two during early development, identical twins will result. This does not happen in protostomes.
[break]
In deuterostomes, the coelom typically forms through enterocoelous development. In enterocoelous development, the mesoderm of the gastrula forms as two outgrowths of the endoderm. As they grow larger, the two mesodermal pouches grow toward each other. Eventually, they meet and merge. The space between the mesodermal layers forms the embryonic coelom.
http://www.freethought-forum.com/forum/gallery/files/5/0/enterocoelous_original.jpg
Enterocoelous development: The mesoderm forms as two outgrowths of
endodermal tissue. The mesodermal pouches grow toward each other as
they enlarge. Eventually, the two mesodermal pouches meet and fuse.
The space between the mesodermal layers forms the embryonic coelom.
[break]
In deuterostomes, the blastopore forms the anus. Later in development, a second opening forms at the other end of the digestive tube and becomes the mouth.
http://www.freethought-forum.com/forum/gallery/files/5/0/development_524412.jpg
A summary of the developmental differences between protostomes and deuterostomes.
[break=New Views of Animal Phylogeny]
[b]New Views of Animal Phylogeny:
In the past, animals were generally classified according to their body plans. For instance, it was assumed that all animals with a pseudocoelom were probably closely-related. But organisms with similar body plans may not resemble each other because of common ancestry; it may be a result of convergent evolution. So it’s dangerous to assume that just because two different species have similar body plans that they must be close relatives.
Nowadays, we can directly compare the biomolecules (principally proteins) that make up organisms’ bodies, and we can directly compare organisms’ DNA sequences. These molecular techniques allow us to reconstruct animals’ phylogenies with much more certainty than was possible in the past. In some cases, these techniques have shown that animals which appear to be close relatives because of similar body plans are, in fact, only distantly related to each other.
Remember that, when constructing a classification scheme, biologists are hoping to classify organisms into clades. That is, we’re hoping to construct taxa that accurately show evolutionary histories and relationships – phylogenies. As more molecular data are gathered, our understanding of the relationships between animal taxa will continue to be refined, which means that classifications sometimes change in the light of new data.
This means that you shouldn’t think of the phylogenies found in textbooks as set in stone. As new information comes to light, there will inevitably be changes in how organisms are classified. Even so, on a large scale, we can be fairly certain that our classifications truly represent animals’ relationships. For instance, there are about three dozen currently-recognized animal phyla, and that is not likely to change very much. On the other hand, on a smaller scale, there are plenty of animal groups whose phylogenies are still the subject of investigation and of lively debate within the scientific community.
http://www.freethought-forum.com/forum/gallery/files/5/0/traditional.jpg
A “traditional” animal phylogeny, based on comparison of body plans.
(Many minor phyla have been omitted, for clarity.)
The “traditional” way of constructing animal phylogenies was through comparison of body plans. According to this classification technique, there are two main groups of animals, the Parazoa and the Eumetazoa. The parazoans are the sponges (Phylum Porifera) and similar animals. Their defining characteristic is that they lack true tissues. All other animals have true tissues and are traditionally classified as eumetazoans.
In traditional phylogenies, the Eumetazoa are further divided into the Radiata and the Bilateria. The radiates are the jellies, sea anemones and corals (Phylum Cnidaria), the comb jellies (Phylum Ctenophora) and similar animals. Their defining characteristics are that they’re diploblastic and have radial symmetry. The Bilateria are triploblastic and have bilateral symmetry.
Traditional phylogenies subdivide the Bilateria into the Acoelomates, the Pseudocoelomates, and the Coelomates. The Acoelomates include the flatworms (Phylum Platyhelminthes) and similar animals. Their defining characteristic, of course, is that each of them lacks an internal, mesoderm-lined body cavity. The Pseudocoelomates include the roundworms (Phylum Nematoda), rotifers (Phylum Rotifera) and similar animals. They have body cavities, but they aren’t entirely lined by mesodermal tissue and are not subdivided. The Coelomates include animals as diverse as insects (Phylum Arthropoda), sea stars (Phylum Echinodermata), and humans (Phylum Chordata). Their defining characteristic is the possession of an internal body cavity that is entirely lined by mesodermal tissue and can be subdivided.
Traditionally, the Coelomate animals are further subdivided into the Protostomia and the Deuterostomia. Most animals are protosomes; the two major deuterostome phyla are the echinoderms and the chordates.
Some animals, such as the Lophophorates, are notoriously difficult to classify using traditional techniques. Molecular techniques allow us to classify them with far greater certainty. They also show that some of the traditional taxa do not accurately reflect animals’ phylogenies.
[break=Molecular Phylogenies]
http://www.freethought-forum.com/forum/gallery/files/5/0/animal_phylogeny.jpg
A more recent animal phylogeny, based on molecular data.
Molecular data shows that some of the “traditional” taxa are true clades, while others are only grades and are thus in need of revision. The molecular data also strongly suggest that the choanoflagellate protists were the ancestors of all members of the Kingdom Animalia.
According to the molecular data, the Parazoa and the Eumetazoa are true clades. Evidently, animals split into these two groups very early in their history.
The Radiata and the Bilateria are also true clades, according to the molecular data. This is another split that occurred very early in animal history.
Things get more complicated after this point, however, as the molecular data show that the Acoelomates, Pseudocoelomates and Coelomates are not true clades. As such, though the terms are sometimes used to describe animals, they’re no longer used to classify them, since they don’t accurately indicate phylogenies.
The molecular data indicate that the division of the Bilateria into the Protostomia and the Deuterostomia is justified and that these are true clades. Again, the separation between protostomes and deuterostomes occurred very early in animal history, when the most complicated living animals would have resembled worms. (That’s why that episode of Star Trek: The Next Generation in which Lieutenant Barclay “devolved” into a spider-like creature is so funny. Spiders are protostomes; humans are deuterostomes. Therefore, humans most-definitely are [b]not descended from anything that ever looked even [b]remotely like a spider!)
The molecular data show that the Protostomia are divided into two major groups, the Lophotrochozoa and the Ecdysozoa. We’ll discuss each of these groups in later chapters.
Finally, the molecular data show that, as had been suspected from morphological and developmental evidence, the two major phyla of the Deuterostomia – the Echinodermata and the Chordata – are indeed closely-related. Perhaps it’s strange to think of a sea star as a fairly close relative of yours, but that is indeed the case.
[b]Points of Agreement:
Though animal phylogenies are continually being revised as more data become available, there are at least five major points that are considered settled beyond any reasonable doubt. As such, these conclusions are unlikely to change in the light of new data.
The first point of agreement is that the animals are indeed a monophyletic group, and that they’re descended from colonial flagellates, apparently the choanoflagellates. In other words, all living animals are descended from a single common ancestor. Animals, therefore, form a true clade that is known as the Metazoa (Kingdom Animalia).
The second point of agreement is that sponges (Phylum Porifera) are basal animals. That is, the ancestors of the sponges split off from the main line of animal evolution very early.
The third point of agreement is that the Eumetazoa (“true animals”) are a real clade. The Eumetazoa, you recall, are the animals that possess true tissues. According to both traditional classification schemes and the molecular data, all eumetazoans – even animals as radically different as jellies and humans – are descended from a common eumetazoan ancestor.
The fourth point of agreement is that the Bilateria (which includes the great majority of animals) and the Radiata (the phyla Cnidaria and Ctenophora) are true clades. Evidently, all of the triploblastic, bilaterally-symmetrical animals are descended from common ancestry. (Some of the Bilateria are not bilaterally-symmetrical as adults, but all are bilaterally-symmetrical as embryos.) Similarly, the diploblastic, radially-symmetrical animals are descended from common ancestry.
The final point of agreement is that, within the Bilateria, the Protostomia and the Deuterostomia represent true clades. Traditional and molecular phylogenies sometimes disagree on which animals are protostomes and which are deuterostomes, but the molecular data clearly indicate that the Protostomia and the Deuterostomia are real clades.
[break]
By now, we have a basic understanding of how animals develop and of how they’re classified. We also have a basic understanding animal diversity and of the major stages in animal evolution. In the next chapter, we will return to the subject of animal reproduction, to explore it in a little more detail.