The Lone Ranger
11-29-2008, 01:24 AM
An Introduction to Zoology
Chapter 10: The Radiata: Phylum Ctenophora:
The Phylum Ctenophora:
In the previous chapter, we discussed the Phylum Cnidaria, one of the two phyla of diploblastic (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=5), radially-symmetrical (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=3#content_start) animals. The other radiate phylum is the Phylum Ctenophora (from the Greek “ktenos,” meaning “comb,” and “phora,” meaning “bearing”). Ctenophorans are commonly known as “sea walnuts” or “comb jellies.” All known ctenophoran species are marine, and ctenophorans are the largest organisms that use cilia as their main source of propulsion.
Ctenophorans are fragile, beautiful creatures. In most species, the body is transparent and looks like glass. The comb rows create iridescent patterns as they beat. In addition, many species are bioluminescent, and can produce spectacular light displays. One of the most memorable experiences I’ve ever had was a night of sitting on a dock and watching comb jellies swimming in the water below, their comb plates glowing brightly. It was on an Ecology class trip, and several students desperately wanted to catch some of them to take home. I had a hard time convincing them that the fragile creatures would never survive, and so should be left alone.
As you might expect, the ctenophorans and the cnidarians have a lot in common. The ctenophorans are a much smaller phylum, however, and are much less familiar to most people. I suspect that the average person, upon seeing a ctenophoran, would think that it was a kind of cnidarian. Nonetheless, despite the superficial similarities between ctenophorans and cnidarians, there are a number of important differences between them.
[b]Ctenophoran Characteristics:
The defining characteristic is that these animals have eight rows of modified cilia known as “combs” (properly, ctenes) arranged around their bodies. These comb rows are used for swimming in most species, though some species creep along the ocean floor instead.
Ctenophorans have a modified form of radial symmetry (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=3#content_start) known as biradial symmetry. The arrangement of their internal canals and of their paired tentacles produces a unique combination of radial and bilateral symmetry (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=4). The body is usually ellipsoid or spherical in shape, with oral (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=4) and aboral (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=4) ends, but no head.
Between their epidermis (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=4) and gastrodermis (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=4), an adult ctenophoran has a non-living gelatinous substance known as collenchyme, which is similar to the mesoglea (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=4) of cnidarians. An important difference is that the collenchyme often has muscle cells embedded in it. While ctenophores are not truly triploblastic (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=6) and do not have true mesodermal tissues, their bodies are somewhat more complex than are those of most cnidarians.
Unlike a cnidarian, a ctenophoran has a complete (one-way) gut. That is, the animal has a mouth at one end of the body, where food is ingested, and a pair of anal pores at the other end of the body, where (some) undigested wastes are expelled. You may recall that a cnidarian, in contrast, has a single opening into its digestive cavity, which functions as both the mouth and the anus. This is another way in which ctenophorans have rather more complex bodies than do most cnidarians.
Most ctenophorans have a pair of tentacles, which are used in gathering food. These tentacles lack cnidocytes (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=5#content_start), but they do have specialized cells known as colloblasts or “glue cells.”
Like cnidarians, ctenophorans have muscle cells that aid in movement. As in cnidarians, these cells are not organized into true muscles, however.
Like cnidarians, ctenophorans have neurons organized into a network that allows them to coordinate their movements and respond to changes in their environments. They have no brains, however. In many species, a statocyst is present on the aboral surface, which allows them to sense gravity and to orient themselves in the water column.
Ctenophorans reproduce sexually (http://www.freethought-forum.com/forum/showthread.php?t=17235&garpg=2#content_start), and most species are monoecious (http://www.freethought-forum.com/forum/showthread.php?t=17235&garpg=2#content_start) (hermaphroditic). Unlike many cnidarians, there are not separate body types such as polyps (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=6#content_start) and medusae (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=6#content_start). Many species have a distinctive juvenile form known as a cydippid.
Like cnidarians, ctenophorans have neither respiratory nor excretory systems. Ctenophorans also lack any sort of coelomic cavity (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7), being diploblastic (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=5) animals.
http://www.freethought-forum.com/forum/gallery/files/5/0/ctenophoran.jpg
The anatomy of a typical ctenophoran. As you can see, though it’s still a
very simple animal, it’s much more complex than is the average cnidarian.
[b]Comb Rows and Comb Plates:
Ctenophorans have eight equally-spaced bands of modified cilia extending from the aboral pole to the oral pole, ending just before the mouth. Each of these comb rows consists of bands of comb plates. Each comb plate consists of long, fused cilia. Beating of these comb plates propels the animal through the water.
Normally, the beat in each row of comb plates begins at the aboral end and proceeds in sequence along the comb row to the oral end. All eight rows normally beat in unison. This provides forward thrust, and the animal swims forward, mouth-first. If necessary, the animal can reverse the direction in which the plates beat and so swim backwards.
http://www.freethought-forum.com/forum/gallery/files/5/0/comb_plates_original.jpg
A comb jelly showing its eight rows of comb plates. The comb plates scatter light that
strikes them. This results in rapidly-changing color displays as the comb plates beat.
http://www.freethought-forum.com/forum/gallery/files/5/0/combs2.jpg
A greatly-magnified view of a comb jelly’s
comb plate, showing the ctenes.
[b]Ctenophore Anatomy:
As in cnidarians, the outer cell layer develops from the embryonic ectoderm and so is known as the epidermis, while the inner cell layer develops from the embryonic endoderm and is known as the gastrodermis. Also like cnidarians, ctenophorans have a non-living substance filling some of the space between the epidermis and the gastrodermis. In ctenophorans, this gel-like substance is known as collenchyme.
One important difference between the collenchyme of ctenophorans and the mesoglea of cnidarians is that the collenchyme has muscle cells embedded in it. These muscle cells are not organized into true muscles, but their contractions do help to propel the animals through the water.
[b]The Digestive System and Feeding:
Comb jellies are sometimes said to have a “sac within a sac” body plan. That’s because the outer body wall (the epidermis) is one sac-like layer of tissue, and within it is a second sac-like layer of tissue, the gastrodermis. The gastrodermis forms the animal’s gut.
An important difference between the gut of a ctenophoran and the gut of a cnidarian is that a ctenophoran has a complete (one-way) gut. In other words, food enters at one end (the mouth), travels in one direction, and then is expelled at the other end (the anal pores). The gut of a ctenophoran is also much more complex than is the simple gastrovascular cavity (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=4#content_start) of a cnidarian. A ctenophoran’s gut is not completely one-way, however. Smaller bits of undigested matter are expelled from the anal pore, but some material is expelled from the mouth as well.
A comb jelly feeds either by ingesting food as it swims forward or by capturing prey with its tentacles and then pushing the prey into its mouth. After food enters a comb jelly’s mouth, it passes into a short tube known as the pharynx before entering the stomach. Cells lining the pharynx and stomach secrete digestive enzymes to digest the food. From the stomach, extensions of the gut extend into the collenchyme and branch out to the comb plates, to the bases of the tentacles, and to other regions of the body. Because no part of the animal’s body is very far from the gut molecules of digested food can diffuse to any cell in the body. This means that no circulatory system is needed to transport digested food to the various body regions. Undigested material is expelled either through a pair of anal pores or the mouth.
[b]Tentacles and Colloblasts:
Like cnidarians, ctenophorans possess extensions of the body wall known as tentacles. But where most cnidarians have numerous tentacles, a ctenophoran usually has only two. These tentacles can be retracted into tentacle sheaths when the animal isn’t feeding.
The tentacles of a ctenophoran bear specialized cells known as colloblasts or “glue cells.” As the common name implies, they produce a sticky substance that traps and holds any small animal unfortunate enough to come into contact with a comb jelly’s tentacles. Captured prey is then brought to the comb jelly’s mouth and ingested.
http://www.freethought-forum.com/forum/gallery/files/5/0/pleurobrachia_pileus.jpg
The ctenophoran Pleurobrachia pileus
(commonly known as “Sea Gooseberry”)
and its remarkable tentacles.
[b]Nervous and Sensory Tissues:
The nervous system of a ctenophoran is similar to that of a cnidarian. It consists of neurons (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=8#content_start) just under the epidermis that are organized into a neural net. These neurons are not organized into any kind of brain, but they are concentrated under each comb row, and apparently help to organize and control the beating of the ctenes.
At its aboral end, a comb jelly has an apical sense organ containing a statocyst. The statocyst is a hollow cavity, the inside of which is lined by cilia. Inside the cavity is a hard crystal of calcium carbonate known as a statolith. Since the statolith is denser than the surrounding water, as the comb jelly changes orientation, the statolith rolls or falls until it comes to rest on the cilia lining the statocyst cavity. Since these cilia are touch-sensitive, this gives the comb jelly a means of distinguishing “up” from “down.”
The neurons under the epidermis make a ctenophoran very sensitive to touch. The tentacles are especially touch-sensitive, and will withdraw into their sheaths if touched.
http://www.freethought-forum.com/forum/gallery/files/5/0/statocyst.jpg
The structure of a statocyst. Note that as the animal
changes orientation, the statolith inside the statocyst
cavity moves, triggering sensory cilia in the process.
[b]Reproduction and Development:
Ctenophorans reproduce sexually, but unlike cnidarians, most ctenophorans are hermaphroditic. Many species have a distinctive, free-swimming juvenile form known as a cydippid. A cydippid gradually develops into an adult, without metamorphosis (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=12#content_start).
http://www.freethought-forum.com/forum/gallery/files/5/0/cydippid.jpg
A juvenile ctenophoran, or cydippid.
[b]Respiratory and Excretory Systems:
In a ctenophoran, as in a cnidarian, every body cell is in contact with the surrounding sea water. Because of this, oxygen can diffuse directly into body cells from the surrounding water and CO2 and other metabolic wastes can diffuse from body cells and into the surrounding water. For this reason, ctenophorans do not need respiratory systems to transport oxygen into body tissues and CO2 out of them. Neither do ctenophorans need excretory systems to rid their bodies of metabolic wastes such as urea and ammonia.
[b]Ctenophorans and Homeobox Genes:
For such simple animals, comb jellies have remarkably complex genetic makeups. In particular, their homeobox (Hox) genes are much more complex than most people would expect.
Homeobox genes are found in all eumetazoans (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=19), and are important in regulating the developmental process. In most animals, these genes help determine the animal’s anterior/posterior (http://www.freethought-forum.com/forum/showthread.php?t=17203&garpg=2#content_start) axis and so determine the positions of the head, brain, heart, limbs, and rear end. They’re what allow a hollow ball of undifferentiated cells (a blastula (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=11#content_start)) to ultimately develop into a complex animal.
Given that a comb jelly has no head, no brain, no heart, and no limbs, you’d therefore expect that its homeobox genes don’t resemble those of “advanced” animals very much. As it turns out, though, the homeobox genes of ctenophorans are remarkably similar to those found in you and me. This indicates that homeobox genes evolved very early in the history of the animal kingdom, though we still don’t know exactly what comb jellies use them for.
[break=What Comes Next]
The cnidarians and the ctenophorans are very different from the remaining eumetazoans. From now on, all of the animals we’ll be discussing are members of the Bilateria (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=19#content_start). All of these animals are triploblastic and bilaterally symmetrical, at least embryonically. We’ll begin our discussion of the Bilateria in the next chapter.
Chapter 10: The Radiata: Phylum Ctenophora:
The Phylum Ctenophora:
In the previous chapter, we discussed the Phylum Cnidaria, one of the two phyla of diploblastic (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=5), radially-symmetrical (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=3#content_start) animals. The other radiate phylum is the Phylum Ctenophora (from the Greek “ktenos,” meaning “comb,” and “phora,” meaning “bearing”). Ctenophorans are commonly known as “sea walnuts” or “comb jellies.” All known ctenophoran species are marine, and ctenophorans are the largest organisms that use cilia as their main source of propulsion.
Ctenophorans are fragile, beautiful creatures. In most species, the body is transparent and looks like glass. The comb rows create iridescent patterns as they beat. In addition, many species are bioluminescent, and can produce spectacular light displays. One of the most memorable experiences I’ve ever had was a night of sitting on a dock and watching comb jellies swimming in the water below, their comb plates glowing brightly. It was on an Ecology class trip, and several students desperately wanted to catch some of them to take home. I had a hard time convincing them that the fragile creatures would never survive, and so should be left alone.
As you might expect, the ctenophorans and the cnidarians have a lot in common. The ctenophorans are a much smaller phylum, however, and are much less familiar to most people. I suspect that the average person, upon seeing a ctenophoran, would think that it was a kind of cnidarian. Nonetheless, despite the superficial similarities between ctenophorans and cnidarians, there are a number of important differences between them.
[b]Ctenophoran Characteristics:
The defining characteristic is that these animals have eight rows of modified cilia known as “combs” (properly, ctenes) arranged around their bodies. These comb rows are used for swimming in most species, though some species creep along the ocean floor instead.
Ctenophorans have a modified form of radial symmetry (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=3#content_start) known as biradial symmetry. The arrangement of their internal canals and of their paired tentacles produces a unique combination of radial and bilateral symmetry (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=4). The body is usually ellipsoid or spherical in shape, with oral (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=4) and aboral (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=4) ends, but no head.
Between their epidermis (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=4) and gastrodermis (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=4), an adult ctenophoran has a non-living gelatinous substance known as collenchyme, which is similar to the mesoglea (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=4) of cnidarians. An important difference is that the collenchyme often has muscle cells embedded in it. While ctenophores are not truly triploblastic (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=6) and do not have true mesodermal tissues, their bodies are somewhat more complex than are those of most cnidarians.
Unlike a cnidarian, a ctenophoran has a complete (one-way) gut. That is, the animal has a mouth at one end of the body, where food is ingested, and a pair of anal pores at the other end of the body, where (some) undigested wastes are expelled. You may recall that a cnidarian, in contrast, has a single opening into its digestive cavity, which functions as both the mouth and the anus. This is another way in which ctenophorans have rather more complex bodies than do most cnidarians.
Most ctenophorans have a pair of tentacles, which are used in gathering food. These tentacles lack cnidocytes (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=5#content_start), but they do have specialized cells known as colloblasts or “glue cells.”
Like cnidarians, ctenophorans have muscle cells that aid in movement. As in cnidarians, these cells are not organized into true muscles, however.
Like cnidarians, ctenophorans have neurons organized into a network that allows them to coordinate their movements and respond to changes in their environments. They have no brains, however. In many species, a statocyst is present on the aboral surface, which allows them to sense gravity and to orient themselves in the water column.
Ctenophorans reproduce sexually (http://www.freethought-forum.com/forum/showthread.php?t=17235&garpg=2#content_start), and most species are monoecious (http://www.freethought-forum.com/forum/showthread.php?t=17235&garpg=2#content_start) (hermaphroditic). Unlike many cnidarians, there are not separate body types such as polyps (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=6#content_start) and medusae (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=6#content_start). Many species have a distinctive juvenile form known as a cydippid.
Like cnidarians, ctenophorans have neither respiratory nor excretory systems. Ctenophorans also lack any sort of coelomic cavity (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7), being diploblastic (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=5) animals.
http://www.freethought-forum.com/forum/gallery/files/5/0/ctenophoran.jpg
The anatomy of a typical ctenophoran. As you can see, though it’s still a
very simple animal, it’s much more complex than is the average cnidarian.
[b]Comb Rows and Comb Plates:
Ctenophorans have eight equally-spaced bands of modified cilia extending from the aboral pole to the oral pole, ending just before the mouth. Each of these comb rows consists of bands of comb plates. Each comb plate consists of long, fused cilia. Beating of these comb plates propels the animal through the water.
Normally, the beat in each row of comb plates begins at the aboral end and proceeds in sequence along the comb row to the oral end. All eight rows normally beat in unison. This provides forward thrust, and the animal swims forward, mouth-first. If necessary, the animal can reverse the direction in which the plates beat and so swim backwards.
http://www.freethought-forum.com/forum/gallery/files/5/0/comb_plates_original.jpg
A comb jelly showing its eight rows of comb plates. The comb plates scatter light that
strikes them. This results in rapidly-changing color displays as the comb plates beat.
http://www.freethought-forum.com/forum/gallery/files/5/0/combs2.jpg
A greatly-magnified view of a comb jelly’s
comb plate, showing the ctenes.
[b]Ctenophore Anatomy:
As in cnidarians, the outer cell layer develops from the embryonic ectoderm and so is known as the epidermis, while the inner cell layer develops from the embryonic endoderm and is known as the gastrodermis. Also like cnidarians, ctenophorans have a non-living substance filling some of the space between the epidermis and the gastrodermis. In ctenophorans, this gel-like substance is known as collenchyme.
One important difference between the collenchyme of ctenophorans and the mesoglea of cnidarians is that the collenchyme has muscle cells embedded in it. These muscle cells are not organized into true muscles, but their contractions do help to propel the animals through the water.
[b]The Digestive System and Feeding:
Comb jellies are sometimes said to have a “sac within a sac” body plan. That’s because the outer body wall (the epidermis) is one sac-like layer of tissue, and within it is a second sac-like layer of tissue, the gastrodermis. The gastrodermis forms the animal’s gut.
An important difference between the gut of a ctenophoran and the gut of a cnidarian is that a ctenophoran has a complete (one-way) gut. In other words, food enters at one end (the mouth), travels in one direction, and then is expelled at the other end (the anal pores). The gut of a ctenophoran is also much more complex than is the simple gastrovascular cavity (http://www.freethought-forum.com/forum/showthread.php?t=18474&garpg=4#content_start) of a cnidarian. A ctenophoran’s gut is not completely one-way, however. Smaller bits of undigested matter are expelled from the anal pore, but some material is expelled from the mouth as well.
A comb jelly feeds either by ingesting food as it swims forward or by capturing prey with its tentacles and then pushing the prey into its mouth. After food enters a comb jelly’s mouth, it passes into a short tube known as the pharynx before entering the stomach. Cells lining the pharynx and stomach secrete digestive enzymes to digest the food. From the stomach, extensions of the gut extend into the collenchyme and branch out to the comb plates, to the bases of the tentacles, and to other regions of the body. Because no part of the animal’s body is very far from the gut molecules of digested food can diffuse to any cell in the body. This means that no circulatory system is needed to transport digested food to the various body regions. Undigested material is expelled either through a pair of anal pores or the mouth.
[b]Tentacles and Colloblasts:
Like cnidarians, ctenophorans possess extensions of the body wall known as tentacles. But where most cnidarians have numerous tentacles, a ctenophoran usually has only two. These tentacles can be retracted into tentacle sheaths when the animal isn’t feeding.
The tentacles of a ctenophoran bear specialized cells known as colloblasts or “glue cells.” As the common name implies, they produce a sticky substance that traps and holds any small animal unfortunate enough to come into contact with a comb jelly’s tentacles. Captured prey is then brought to the comb jelly’s mouth and ingested.
http://www.freethought-forum.com/forum/gallery/files/5/0/pleurobrachia_pileus.jpg
The ctenophoran Pleurobrachia pileus
(commonly known as “Sea Gooseberry”)
and its remarkable tentacles.
[b]Nervous and Sensory Tissues:
The nervous system of a ctenophoran is similar to that of a cnidarian. It consists of neurons (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=8#content_start) just under the epidermis that are organized into a neural net. These neurons are not organized into any kind of brain, but they are concentrated under each comb row, and apparently help to organize and control the beating of the ctenes.
At its aboral end, a comb jelly has an apical sense organ containing a statocyst. The statocyst is a hollow cavity, the inside of which is lined by cilia. Inside the cavity is a hard crystal of calcium carbonate known as a statolith. Since the statolith is denser than the surrounding water, as the comb jelly changes orientation, the statolith rolls or falls until it comes to rest on the cilia lining the statocyst cavity. Since these cilia are touch-sensitive, this gives the comb jelly a means of distinguishing “up” from “down.”
The neurons under the epidermis make a ctenophoran very sensitive to touch. The tentacles are especially touch-sensitive, and will withdraw into their sheaths if touched.
http://www.freethought-forum.com/forum/gallery/files/5/0/statocyst.jpg
The structure of a statocyst. Note that as the animal
changes orientation, the statolith inside the statocyst
cavity moves, triggering sensory cilia in the process.
[b]Reproduction and Development:
Ctenophorans reproduce sexually, but unlike cnidarians, most ctenophorans are hermaphroditic. Many species have a distinctive, free-swimming juvenile form known as a cydippid. A cydippid gradually develops into an adult, without metamorphosis (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=12#content_start).
http://www.freethought-forum.com/forum/gallery/files/5/0/cydippid.jpg
A juvenile ctenophoran, or cydippid.
[b]Respiratory and Excretory Systems:
In a ctenophoran, as in a cnidarian, every body cell is in contact with the surrounding sea water. Because of this, oxygen can diffuse directly into body cells from the surrounding water and CO2 and other metabolic wastes can diffuse from body cells and into the surrounding water. For this reason, ctenophorans do not need respiratory systems to transport oxygen into body tissues and CO2 out of them. Neither do ctenophorans need excretory systems to rid their bodies of metabolic wastes such as urea and ammonia.
[b]Ctenophorans and Homeobox Genes:
For such simple animals, comb jellies have remarkably complex genetic makeups. In particular, their homeobox (Hox) genes are much more complex than most people would expect.
Homeobox genes are found in all eumetazoans (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=19), and are important in regulating the developmental process. In most animals, these genes help determine the animal’s anterior/posterior (http://www.freethought-forum.com/forum/showthread.php?t=17203&garpg=2#content_start) axis and so determine the positions of the head, brain, heart, limbs, and rear end. They’re what allow a hollow ball of undifferentiated cells (a blastula (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=11#content_start)) to ultimately develop into a complex animal.
Given that a comb jelly has no head, no brain, no heart, and no limbs, you’d therefore expect that its homeobox genes don’t resemble those of “advanced” animals very much. As it turns out, though, the homeobox genes of ctenophorans are remarkably similar to those found in you and me. This indicates that homeobox genes evolved very early in the history of the animal kingdom, though we still don’t know exactly what comb jellies use them for.
[break=What Comes Next]
The cnidarians and the ctenophorans are very different from the remaining eumetazoans. From now on, all of the animals we’ll be discussing are members of the Bilateria (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=19#content_start). All of these animals are triploblastic and bilaterally symmetrical, at least embryonically. We’ll begin our discussion of the Bilateria in the next chapter.