An Introduction to Zoology: Chapter 11
 

An Introduction to Zoology

Chapter 11: The Bilateria: Acoelomate Lophotrochozoans:

The Bilateria:

In the two previous chapters, we discussed the phyla Cnidaria and Ctenophora, the diploblastic, radially-symmetrical animals that are collectively known as the Radiata. All other animals are triploblastic and bilaterally symmetrical, and so are collectively known as the Bilateria.

All of the Bilateria have elongated, bilaterally-symmetrical bodies, at least during the embryo stage. Some of the Bilateria are radially-symmetrical as adults, it’s true, but all of them develop from bilaterally-symmetrical embryos. Most of the Bilateria also show some degree of cephalization, meaning that their sense organs are concentrated at the anterior end to form a head.




The remaining articles in this series will focus on the various phyla within the clade Bilateria. First, though, we’ll briefly discuss bilaterian classification and diversity.

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Acoelomates, Pseudocoelomates, and Coelomates:

Because it is triploblastic, a bilaterian has the capacity to form a coelom. You may recall that a true coelom is a fluid-filled (occasionally, an air-filled) body cavity that is entirely lined by mesodermal tissue. Obviously, only an organism that actually has mesoderm can have a true coelom, so radiate animals cannot be coelomates. You may also recall that many internal organs are derived from mesodermal tissue, so triploblastic animals generally have much more complex bodies than do diploblastic animals.

We often divide the Bilateria into the acoelomates, the pseudocoelomates, and the coelomates, according to what kind of coelom (if any) is present. These terms are simply descriptions of how animals’ bodies are built; they do not necessarily indicate evolutionary relationships.

An acoelomate is any triploblastic animal that has no internal, fluid-filled body cavity. (“A” means “without,” so an “acoelomate” is “without a coelom.”) In an acoelomate, the space between the ectoderm and the endoderm is occupied by organs derived from mesodermal tissue, plus a loose collection of mesodermally-derived cells called parenchyma.


You may recall that, since water is incompressible, a coelom, if present, can form a hydrostatic skeleton that helps its possessor resist external pressure. A hydrostatic skeleton can also be used to help redirect force produced by muscle contractions. Because they lack internal skeletons of any sort, few acoelomates are capable of burrowing, since they cannot resist external pressure. Similarly, few acoelomates can generate sufficient muscle force to be capable of swimming.

Because they cannot resist external pressure, most acoelomates are pressed flat by a combination of the downward pull of gravity and the pressure exerted by the surrounding atmosphere or water. That’s why these animals are frequently referred to as “flatworms.”

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A pseudocoelomate is any triploblastic animal that has a “false coelom.” (“Pseudo” means “false.”) A pseudocoelom is an internal, fluid-filled cavity that lies between endodermal and mesodermal tissues, instead of being completely surrounded by mesodermal tissue. Because it is not entirely surrounded by mesodermal tissue, a pseudocoelom cannot be subdivided and compartmentalized the way that a true coelom can.

Since a pseudocoelomate has an internal hydrostatic skeleton, it can resist external pressure, and it can use the hydrostatic skeleton to help redirect muscle-contraction force. Because of this, many pseudocoelomates are adept at burrowing, and some can penetrate surprisingly-dense substances. Many are also capable of swimming. Because their bodies are not flattened by external pressure, the best-known of the pseudocoelomates are commonly called “roundworms.”

One disadvantage of a pseudocoelom is that, because it is not compartmentalized, a pseudocoelomate cannot manipulate different parts of its body with anywhere near the fine degree of control that most coelomates can accomplish. Those pseudocoelomates that swim via muscular contractions move through the water with a characteristically jerky and easily-recognized “thrashing” motion. It’s inelegant, to be sure, but it’s still a much more efficient means of movement than any acoelomate can manage.

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A coelomate (or eucoelomate) is any triploblastic animal that has a true coelom. A true coelom is entirely surrounded by mesodermal tissue, and so can be subdivided and compartmentalized, unlike a pseudocoelom.

Since it has an internal hydrostatic skeleton, a coelomate, like a pseudocoelomate, can resist external pressure. And because the coelom is usually subdivided and compartmentalized, a coelomate is usually capable of a far greater degree of control over its body movements than a pseudocoelomate can manage. This is because a coelomate, unlike a pseudocoelomate, can manipulate different body sections independently. Another advantage of having a compartmentalized body cavity is that infection or damage to one portion of the body is much less likely to prove fatal. Any disease-causing organism that manages to invade a coelomate’s body will find it difficult to move from one body compartment to another, and so may not be able to cause fatal damage to the organism. Similarly, if a foreign object penetrates a coelomate’s skin, the entire body cavity will not lose pressure as would be the case in a pseudocoelomate; thus a wound that would be fatal to a pseudocoelomate might well prove only an inconvenience to a coelomate of the same size.

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Bilaterian Diversity: Protostomes and Deuterostomes:

While it’s often convenient to divide the Bilateria into “acoelomates,” “pseudocoelomates,” and “coelomates,” this is not a “natural” classification scheme, since it doesn’t actually tell you much about the evolutionary relationships within the Bilateria. If we go by evolutionary relationships, there are two major bilaterian clades, the Protostomata and the Deuterostomata. The Bilateria split into the protostomes and the deuterostomes a very long time ago – at the time, no animal more complex than a worm had yet evolved.

The great majority of the Bilateria are protostomes. As you no-doubt recall, protostomes are triploblastic animals in which the blastopore develops into the mouth, and the anus (if present) forms later. Most protostomes have spiral and determinate cleavage. Additionally, formation of the mesoderm in most protostomes occurs through schizocoelous development.

The deuterostomes make up a rather small minority of bilaterian species. We tend to think of them as important, however, since we happen to be deuterostomes. Deuterostomes are triploblastic animals in which the blastopore develops into the anus, and the mouth forms later. Most deuterostomes have radial and indeterminate cleavage. In most deuterostomes, formation of the mesoderm occurs through enterocoelous development.

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Protostome Diversity: Lophotrochozoans and Ecdysozoans:

We’ll get to the deuterostomes eventually, but we’ll start our survey of bilaterian diversity with the protostomes. Just as the Bilateria can be divided into two great clades, so can the protostomes be subdivided into two distinct clades: these are the clades Lophotrochozoa and Ecdysozoa.



The members of the clade Lophotrochozoa are very diverse indeed, but molecular analyses show that they’re all descended from common ancestry, and so they are a true clade. These animals share a distinctive, horseshoe-shaped feeding structure called a lophophore, and/or a distinctive larval form known as a trochophore.


In those animals that have one, a lophophore is a horseshoe-shaped “crown” of ciliated tentacles that surrounds the mouth. The beating of the cilia creates water currents that help to draw food into a lophophoran’s mouth.


A trochophore is a distinctive, easily-recognized larval form found in some lophotrochozoans. Trochophore larvae are tiny, translucent, and in most species, more or less top-shaped. A trochophore has one or more distinctive bands of cilia that it uses to propel itself through the water. Many aquatic lophotrochozoans have trochophore larvae, including many members of the phyla Annelidae and Mollusca. Some members of the phyla Platyhelminthes, Nemertea, Echiura, and others also have trochophore or trochophore-like larvae.

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The members of the clade Ecdysozoa are an even more diverse group than are the Lophotrochozoa. Like the lophotrochozoans, however, the ecdysozoans appear to be a genuine clade. What distinguishes the ecdysozoans is that they have an outer, non-living cuticle that functions as an external skeleton (an exoskeleton).

The cuticle is secreted by an ecdysozoan’s epidermis (skin), and covers the body, providing a more or less rigid structure that provides protection against injury and, in many species, against dehydration. In many species, the cuticle also serves as an attachment-site for muscles, allowing for more efficient movement.

One disadvantage of a cuticle is that since it is not made of living tissue, it does not grow with the animal. This means that it must be shed (molted) on occasion, to permit its owner’s growth.


The ecdysozoans are a much larger and more diverse group of animals than are the lophotrochozoans. The lophotrochozoans, in addition to being a smaller group, tend to have rather simpler body plans. So, we’ll begin our discussion of bilaterian diversity with the lophotrochozoans. Specifically, we’ll begin with the simplest of the lophotrochozoans, the acoelomates.

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Acoelomate Animals: Flatworms, Mesozoans, and Ribbon Worms:

There are four major lophotrochozoan phyla in which no fully-developed coelom is present. These are the phyla Acoelomorpha, Platyhelminthes, Mesozoa, and Nemertea. These are the simplest animals with bilateral symmetry and cephalization.

In most of these animals, there is no coelom of any kind. The exception is the Phylum Nemertea; these animals do have a coelom of sorts. The “coelom” of a nemertean is present only in the anterior portion of the animal’s body, though, and it appears to have evolved independently of the true coelom found in the eucoelomate animals. Except for the presence of a “coelom,” a nemertean is much more similar to a platyhelminth than it is to a eucoelomate. As such, it seems more or less certain that this shared characteristic of nemerteans and eucoelomates is due to convergent evolution rather than common ancestry. For that reason, we’ll consider the nemerteans with the platyhelminthes and other acoelomate animals.

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Phylum Acoelomorpha:

Members of the phyla Acoelomorpha and Platyhelminthes are commonly known as “flatworms.” The “flat” part is self-explanatory, but what is a “worm”?

Well, the acoelomorphs are the first animal phylum we’ve discussed whose members have elongated, bilaterally-symmetrical bodies. They’re also the first animals we’ve discussed that show any degree of cephalization. These are two important characteristics of “worms.”

A typical cnidarian or ctenophoran isn’t especially active. For such an animal, radial symmetry seems to work just fine, since a radially-symmetrical animal can sense danger (or prey) from any direction. The disadvantage is that radially-symmetrical animals are almost invariably slow-moving or completely immobile.

An animal that actively seeks food, shelter, mates, and so forth requires a body that is constructed differently. Active movement is most efficient if the body is elongated, with distinct anterior (front) and posterior (rear) ends. As such, selection for more efficient movement was probably what led to the evolution of bilateral symmetry. An elongated body is most efficient if one surface of the body is specialized for movement. So, the great majority of bilaterally-symmetrical animals have a ventral (belly) surface that is specialized for locomotion, and a dorsal (back) surface where most of the sense organs are located.

Naturally, it’s better to see where you’re going than where you’ve been, so once bilateral symmetry evolved, there would be selection for concentrating the sense organs and neural tissues at the animal’s anterior end. Concentration of neural tissues at the animal’s anterior end allows the formation of a brain of some sort, that can integrate sensory information and coordinate body movements. Having your neural tissues concentrated near the sense organs also allows for quicker responses.

This concentration of neural tissues and sense organs at the anterior end of the animal’s body is cephalization. Virtually all the animals we’ll discuss from now on are bilaterally symmetrical and show at least some degree of cephalization.



“But what is a worm?” I hear you ask. Well, a “worm” is simply any invertebrate animal that has an elongated, limbless body. In modern usage, a “worm” is always an invertebrate, but originally the word referred to any animal with an elongated and more or less legless body. So, if you read old tales, you’ll sometimes see dragons called “worms” (or “wyrms”). Similarly, if you visit the southern Appalachian Mountains, you can still run across people who call rattlesnakes “buzzworms.”

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Acoelomorphan Characteristics:

The acoelomorphs are small, flat worms that are usually less than 5 millimeters in length. Since, as its name implies, an acoelomorph lacks any sort of coelom, its body is flattened. This explains why they’re commonly referred to as “flatworms.”

There are about 350 species of acoelomorphs known, and all known species are aquatic. Some live in brackish water, but most species are marine. Most species are free-living, but some are parasites of larger animals.


Generally speaking, an acoelomorph is a very simple animal, and in some ways it is not a great deal more complex than is a cnidarian. Even so, because an acoelomorph is triploblastic, its internal anatomy is much more complex than is that of a cnidarian or a ctenophoran.

While acoelomorphs have some concentration of neural tissues in the anterior region, there is nothing resembling an actual brain. Neurons in the body are loosely arranged in a radial pattern around the anterior cluster of neurons, instead of being organized into ladder-like pattern as in the platyhelminths.

Like a cnidarian, an acoelomorph has an incomplete (two-way) digestive system. An acoelomorph takes in food through its mouth and passes it back into a simple, sac-like gut. Later, undigested matter is expelled through the mouth. Some acoelomorph species lack a gut entirely.

Also like a cnidarian, an acoelomorph has no excretory, circulatory, or respiratory systems. Since every cell of its tiny, flattened body is within a millimeter or so of the surrounding water, an acoelomorph has no need of an excretory system to expel wastes, a circulatory system to transport substances within its body, or a respiratory system to absorb oxygen and expel CO2.

The sense organs of an acoelomorph are not much more complex than those found in a typical cnidarian. Like many cnidarians, a typical acoelomorph has a gravity-sensing statocyst that functions as an organ of balance, plus one or more light-sensitive ocelli.

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Despite their simplicity, the fact that they possess mesoderm means that acoelomorphs can have much more complex internal structures than can cnidarians or ctenophorans. For instance, acoelomorphs have well-developed muscles, formed from mesodermal tissue. There are three kinds of muscles that a bilaterally-symmetrical animal can possess, and acoelomorphs possess all three.

Longitudinal muscles are arranged parallel to an animal’s long axis. When these muscle are contracted, they tend to cause the animal’s body to shorten. Contracting longitudinal muscles on one side of the body will cause the animal’s body to bend in that direction.

Circular muscles are arranged at right angles to an animal’s long axis. When circular muscles contract, they squeeze the body, causing the animal to grow thinner. Since even an acoelomate is made mostly of water and therefore has a more or less constant volume, if its body grows thinner, it must elongate. So, contraction of circular muscles causes an animal’s body to lengthen.

Diagonal muscles, as you’ve probably guessed, are arranged at distinct angles (but not right angles) to an animal’s long axis. When they contract, they cause the animal’s body to bend and/or twist.

Between them, these muscles allow acoelomorphs far more flexibility and far more control over their movements than is possible for a cnidarian or a ctenophoran.



The reproductive systems of acoelomorphs are also rather more complex than are those of a cnidarian. Acoelomorphs are typically monoecious, so each animal has both male and female reproductive tissues. Instead of simply having gonads that produce spermatozoa and ova, however, an acoelomorph has a series of ducts to conduct the gametes to a common opening (the gonopore), though which they are expelled. An acoelomorph also has various accessory organs that produce a substantial amount of yolk to support the ovum.

Acoelomorphs are the simplest animals that have internal fertilization. All of these specialized reproductive organs mean that reproduction is not nearly as haphazard a process in acoelomorphs as it is in sponges, cnidarians or ctenophorans, almost all of whom have external fertilization.

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Acoelomorphan Development:

There is one thing about the acoelomorphs that is quite distinctive, and that is how they develop. The early cleavage patterns in acoelomorphs differ from those of deuterostomes or other protostomes. Because of this, some researchers believe that the ancestors of modern acoelomorphs split off from the main line of bilaterian evolution before the great split into protostomes and deuterostomes happened. If this is the case, it lends support to the notion that the acoelomorphs are the oldest surviving lineage of bilaterally-symmetrical animals.

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Phylum Platyhelminthes:

Like acoelomorphs, members of the phylum Platyhelminthes are dorsoventrally flattened and so are commonly called “flatworms.” In fact, that’s what “platyhelminthes” means – “platy” means “flat” and “helminthes” means “worms.”

To say that an animal’s body is dorsoventrally flattened means that it is compressed from the top and bottom, not from the sides; in other words, it’s wider than it is tall. A laterally-compressed animal, in contrast, is taller than it is wide.

As you might imagine, acoelomorphs and platyhelminths have a great deal in common. In fact, acoelomorphs were originally classified as members of the Platyhelminthes, before it was discovered that they have a number of important molecular and developmental dissimilarities.

Platyhelminths range from only a millimeter or so in length up to 10 meters or more in length for some tapeworms. Most are only a few centimeters in length, however. Many species are free-living, but quite a lot of modern species are highly-specialized parasites, and these are the ones most people think of when they think of platyhelminths.

“What is a ‘parasite’?” I hear you ask. Well, a parasite is an organism that lives on or inside another organism (called the host) and feeds on its host’s tissues. A free-living organism is one that does not live on or inside another organism.

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Platyhelminth Characteristics:

There is no single, unifying characteristic that defines the Platyhelminthes. For that reason, some suspect that the phylum Platyhelminthes is actually a paraphyletic taxon. For the time being, though, we’ll consider them to be a true clade.

As mentioned, platyhelminths are acoelomate worms with dorsoventrally-flattened bodies. So far, that doesn’t distinguish them from the acoelomorpha. The internal anatomy of platyhelminths tends to be rather more complex than is true of the aceolomorpha, however, and unlike the acoelomorpha, the development of platyhelminths clearly identifies them as protostomes.


Because they lack any sort of waterproof outer covering that would prevent fatal dehydration, platyhelminths must live in water, or at least in wet environments. But whereas all known acoelomorphs live in water, there are a number of terrestrial platyhelminths.


As do acoelomorphs, a platyhelminth has an incomplete digestive system, so the single opening into it functions as both the mouth and the anus. Some parasitic flatworm species lack digestive systems entirely.


Also like acoelomorphs, platyhelminths have true muscles, of all three types. As in the acoelomorphs, this gives them far more control over their movements than cnidarians or ctenophorans can manage, but the lack of any sort of hydrostatic skeleton means that few platyhelminths can burrow, nor are very many species capable of generating enough muscle force to swim.


The sense organs of platyhelminths are generally similar to those of acoelomorphs, and free-living species typically have gravity-sensing statocysts and light-sensitive ocelli. On the other hand, platyhelminths have rather more complex nervous systems than do acoelomorphs; indeed, the platyhelminths are frequently said to be the simplest animals with true nervous systems.

In a typical platyhelminth, neurons are organized into a pair of anterior ganglia that function as a very simple “brain.” (A ganglion is a collection of neural cells.) Extending down the length of the body from each ganglion is a single longitudinal nerve cord. The two nerve cords are connected by transverse nerve cords that bridge them like the rungs on a ladder, so a platyhelminth is said to have a ladder nervous system.

This is a much more centralized and complex nervous system than that possessed by any animal we have yet discussed. Such a nervous system allows for relatively quick and well-coordinated movements and responses, and also allows some degree of learning. The flatworms commonly called “planarians” are frequently used in high school- or college-level biology courses to demonstrate learning, because they can be taught to navigate very simple mazes.


Most species of flatworms are monoecious and reproduce sexually. Nonetheless, many species can reproduce asexually, and some are famous for it. If you did play with planarians in high school or college biology courses, you might have cut one in two. What do you get if you cut a planarian in two? You get two smaller planarians. If you slice one down the midline, the right half grows a new left half and the left half grows a new right half. If you cut one in two crosswise, the anterior end grows a new posterior end and the posterior end grows a new anterior end (including eyespots and anterior ganglia).

Those species of flatworms that are parasitic often have very complex life cycles. We’ll discuss some representative examples shortly.

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Respiration, Circulation, and Excretion:

In free-living platyhelminths, the gut is highly branched and extends throughout the body. This means that no body cell is more than a millimeter or so away from the gut, and so can easily absorb digested food from it. Similarly, the thin, flat body of a platyhelminth means that no body cell is far from the surrounding air or water, so oxygen can diffuse from the air into body tissues while the CO2 produced as a metabolic waste product can easily diffuse out of the animal’s body tissues and into the surrounding air or water.

All of this means that platyhelminths need neither circulatory systems to transport substances within the body nor respiratory systems to move oxygen and carbon dioxide in and out of body tissues. On the other hand, the platyhelminths are the first animals we have discussed that have anything resembling an excretory system.


In most animals, an excretory system has two functions – excretion and osmoregulation. Excretion is the process in which the body rids itself of potentially-toxic metabolic wastes. In platyhelminths, excretion is rarely a problem, since metabolic wastes such as ammonia can simply diffuse out of animals’ bodies. But osmoregulation is an entirely different matter.



If an organism’s cells have a higher solute concentration than does the water it lives in, then the water is said to be hypotonic to the animal’s cells. Of course, if the animal’s cells have a higher solute concentration than does the surrounding water, that means their water concentration is lower. This is a problem for any animal that lives in fresh water.

If cells are surrounded by a hypotonic solution, because the cells have a higher solute concentration (and, therefore, a lower water concentration) than does the solution, water tends to spontaneously diffuse into the cells from the solution. So, freshwater animals have the problem that their cells are constantly absorbing water. If the animal has no way to get rid of this excess water, its cells will absorb water until they burst, killing the animal.

So, freshwater animals must have some means of expelling excess water. The process of regulating the water (or, more precisely, the salt) content of body tissues is known as osmoregulation, and it’s literally a matter of life-and-death for freshwater animals.


If an animal’s cells and the water it lives in have an equal solute concentration, then the water and the animal’s cells are said to be isotonic. Many (but by no means all) marine animals have body tissues that are isotonic to the surrounding seawater. If an animal’s cells are isotonic to the surrounding water, then they neither gain nor lose water. Such an animal does not have to osmoregulate.


Many marine animals must be capable of osmoregulation, however, because their cells have a lower solute concentration than does the surrounding seawater (and, therefore, a higher water concentration). When this is the case, the seawater that the animal lives in is said to be hypertonic to the animal’s tissues. An animal’s cells will lose water to a hypertonic solution, so many marine animals must constantly drink, in order to replace fluid lost to the surrounding sea. In other words, ironic as it seems, one of the biggest problems faced by many marine organisms is dehydation.

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Many freshwater platyhelminths have specialized osmoregulatory structures known as protonephridia. Each protonephridium consists of a tubule that collects excess water as it diffuses into a flatworm’s body; each tubule opens to the outside of the body at one end, and at the other end is a flame cell. A flame cell has numerous flagella that beat constantly and create currents that push excess water out of the animal’s body. (Under a microscope, the beating of the cilia looks like the flickering of a candle flame, hence the name for this specialized type of cell.)

As excess water is pushed out of a flatworm’s body, it also carries away some metabolically-generated ammonia dissolved in it. So protonephridia have both osmoregulatory and excretory functions, and are said to make up a very simple excretory system. As you might expect, many marine flatworms lack protonephridia, since they don’t have the problem of absorbing excess water from their surroundings.

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Platyhelminth Diversity:

There are four currently-recognized classes of platyhelminths. The members of the class Turbellaria are mostly free-living species. These include the familiar planarians. Members of the class Trematoda are commonly called “flukes,” and all known species live as parasites within the bodies of other animals. Members of the class Monogenea are, perhaps confusingly, also called “flukes.” Like the trematodan flukes, all known species of monogenetic flukes are parasites, but where trematodans live inside their hosts’ bodies, monogeneans usually live on the outside surfaces of their hosts. Finally, there are the members of the class Cestoda, commonly known as “tapeworms.”

Personally, I think that many turbellarians are kind of cute. I would guess that nobody thinks a cestode is at all endearing, however.

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Class Turbellaria:

Most turbellarians are free-living, though a few species are parasitic. Most of them are detritivores, meaning that they feed on decomposing organic matter. Some are predators that feed on smaller organisms. Most turbellarians are quite small, only a few millimeters long, but some species can be nearly half a meter long. You’ll find them hiding under cover in marine, freshwater, and moist terrestrial habitats.

A turbellarian has a mouth on its ventral side, and in many species, an organ known as the pharynx can be extended from the mouth and used to slurp up organic matter or smaller organisms. Behind the mouth, the gut divides and subdivides, extending throughout the body. This ensures that no part of the body is very far from the gut, and so can easily absorb digested matter from the gut. Of course, turbellarians have an incomplete (two-way) digestive system, so undigestible matter is expelled from the mouth.


One thing that distinguishes the turbellarians is that the epidermis of the ventral body surface is ciliated. The beating of these cilia allows turbellarians to glide along surfaces, and some of the smaller species can even swim with their cilia.


The best-known turbellarians are animals known as planarians. These animals are common in fresh water, and you can find them gliding along the bottom of just-about any pond or lake. They’re commonly used in high school and college Biology labs in order to demonstrate learning, since a planarian can be taught to navigate a simple maze. They’re also used to demonstrate regeneration; if you cut a planarian in two, each half will regenerate any missing organs and so form a complete animal.

In addition to asexual reproduction through budding, many planarian species can reproduce sexually. Those species that reproduce sexually are usually hermaphroditic.

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Class Trematoda:

Trematodes are commonly known as “flukes,” and all known species are parasitic. Most are endoparasites, meaning that they live inside their hosts’ bodies. A number of trematode species can parasitize humans, causing serious diseases.


Like a lot of parasites, trematodes tend to have complex life cycles. What is meant by this is that a trematode spends part of its life cycle in one host, and part of its life cycle in another host. Some trematodes have as many as five different hosts during their lives. An intermediate host is a host species in which an immature parasite lives. The definitive host is the host species in which the parasite matures and undergoes sexual reproduction.

Why do so many parasite species have such complicated life cycles? It’s widely thought that by dividing their populations between several different host species, parasites reduce the likelihood of extinction. If one host population should be extinguished, the parasites will survive in other host populations.

Even so, reproduction is a perilous thing for most parasites, and parasitic species typically produce simply astonishing numbers of eggs. They must, in order for there to be any chance of some of them being taken up by a compatible host. As such, a typical trematode devotes tremendous amounts of energy to reproduction. It’s not unusual for 80% of a trematode’s body tissues to consist of reproductive organs.


The digestive system of a trematode, when one is present, is similar to that of a turbellarian. Like a turbellarian, a trematode has a two-branched gut that extends throughout the body, but a trematode lacks a turbellarian’s extensible pharynx.

Many trematodes have no digestive system at all. Since many of them live inside the digestive systems of their hosts, they can simply absorb pre-digested matter from their hosts. Since they don’t have to devote any energy or body volume to digestion, this leaves these animals all the more energy and body space to devote to reproduction.


The body of a trematode is usually leaf-shaped, though some are more or less cylindrical in shape. Lacking any sort of coelom, a trematode’s body is flattened to at least some degree, just as is any other platyhelminth’s.


The epidermis of a trematode is syncytial, meaning that it is not divided into individual cells. This distinguishes trematodes from turbellarians. Another distinction between turbellarians and trematodes is that trematodes lack cilia on their ventral surfaces, so they must use their body musculature for movement.


Most trematodes have a pair of suckers on their ventral surfaces, which they use to attach themselves to the insides of their hosts’ body tubes. Typically, there is an oral sucker that surrounds the mouth, and a ventral sucker closer to the animal’s posterior end.

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Human Liver Flukes:

The trematode Clonorchis sinensis (the Chinese Liver Fluke) parasitizes something like 30 million people in eastern Asia. Adult flukes live in the livers of their human hosts, where they reproduce sexually. An infestation of liver flukes can cause serious – sometimes fatal – damage to the host’s liver. Infestation usually occurs from eating improperly-cooked fish. [No, I do not eat sushi; I know far too much about the sorts of things that live in uncooked meat.]


Eggs produced by an adult fluke travel into the host’s digestive system and are eventually shed in the feces. If the feces are deposited into water, they may be eaten by aquatic snails. If an egg is eaten by a snail, a ciliated larval form known as a miracidium hatches from the egg and parasitizes the snail. (The best way to reduce rates of liver fluke infestation is to introduce sanitary plumbing facilities, so that human wastes – and, therefore, liver fluke eggs – don’t wind up in waterways.)


Inside the snail’s body, a miracidium undergoes several metamorphoses. First, it transforms into a sporocyst, then a redia, and finally a tadpole-like cercaria. The larval flukes feed on the snail’s body tissues. Often, they migrate to the snail’s gonads and eat those first. This “castrates” the snail, and since it can no longer devote energy to reproduction, it grows larger. This is good for the larval flukes infesting the snail’s tissues, but not so good for the snail.


Eventually, the cercariae burrow through the snail’s tissues and emerge into the water. They swim through the water until they encounter a fish. Upon encountering a fish, a cercaria burrows through its body wall and into the fish’s muscle tissue. There it undergoes metamorphosis into a metacercaria.


If a human eats an infected fish, metacerecariae migrate up his or her bile duct and into the liver. Inside the liver, they undergo metamorphosis into adult flukes, and the cycle is complete.

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Blood Flukes:

Blood flukes (genus Schistosoma) are common in much of Africa, South and Central America, and eastern Asia. Their life cycles are similar to those of liver flukes, except that they live in their hosts’ blood veins. Infestation by blood flukes causes the disease known as schistosomiasis, and it is a serious problem in the warmer parts of the world. Some 200 million people around the world are infected.

One important difference between blood flukes and liver flukes is that blood flukes are dioecious, whereas liver flukes are monoecious. Male schistosomes are typically larger and broader than are females, and a male has a large groove on his ventral surface known as a gynecophoric canal. The smaller female lives in the male’s gynecophoric canal.


As do larval liver flukes, larval blood flukes have aquatic snails as their intermediate hosts. A person who bathes or swims in infested waters risks infection, because the cercariae can burrow through human skin. As with infestation by liver flukes, the best ways to prevent schistosomiasis are through proper disposal of human wastes and educating people to avoid contaminated water.


Schistosome dermatitis (commonly known as “swimmer’s itch”) is a rash caused by schistosome infection. People who swim in North American lakes are likely to develop swimmer’s itch. Fortunately, the schistosome species that cause it normally affect aquatic birds such as ducks, and cannot survive in humans. When these cercariae penetrate a person’s skin, the person’s immune system quickly destroys them. The redness, swelling and itching that occur with an infection are part of the normal immune response.

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Class Monogenea:

Monogeneans are very similar to trematodes in many ways. Like trematodes, most monogeneans have flattened, leaf-shaped bodies, and are commonly called “flukes.” Also like trematodes, all known species of monogeneans are parasites. An important difference is that monogenetic flukes are typically ectoparasites that live on the outsides of their hosts’ bodies.

Monogenetic flukes typically do much less harm to their hosts than do trematodes, though they attach themselves with hooks instead of suckers. Most species parasitize fishes.

Like trematodes, monogeneans typically have syncytial body surfaces that are not ciliated. So, like trematodes, they must rely on muscular contractions for movement.

An important difference between trematodes and monogeneans is that most monogeneans have simple life styles. That is, a monogenetic fluke has only a single host during its life. A related difference is that monogenetic flukes have direct development. A juvenile monogenean is ciliated and can swim. It swims until it encounters a fish, to which it attaches itself. Over time, the juvenile gradually develops into an adult fluke, with no real process of metamorphosis.

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Class Cestoda:

The cestodes are probably most people’s least-favorite flatworms. Cestodes are highly specialized parasites that live inside their hosts’ digestive systems. They’re commonly known as “tapeworms.”

A tapeworm’s body is almost entirely devoted to reproduction. It can do this because, since it lives inside its host’s digestive system, it has no need of a digestive system of its own. A tapeworm’s body consists of numerous segments, the first of which is known as the scolex. The scolex contains hooks with which it attaches itself to its host’s intestinal wall. If the host animal has many tapeworms living in its intestine, their hooks can do a lot of damage, causing internal bleeding that may result in anemia and even death.

The remainder of a tapeworm’s body is known as the strobila, and it consists of numerous reproductive segments. Each reproductive segment is known as a proglottid. Each proglottid is devoted more or less entirely to production of gametes, and it produces simply prodigious numbers of spermatozoa and ova. Most tapeworm species are hermaphroditic, and a tapeworm is capable of fertilizing its own eggs. Any tapeworm in a person’s gut will be shedding many thousands of eggs per day.

The proglottids, despite their simplicity, do have muscles. Occasionally, a chain of proglottids will break free of the tapeworm and crawl out of the host’s body through the anus. I’ve been told that it’s a somewhat … exciting … experience to have one crawl out of your lower intestine and down your leg.


Like so many other parasites, tapeworms generally have complex life cycles and infect several different hosts. A well-known example is the Beef Tapeworm (Taenia saginata), which has cows as its intermediate host and humans as its definitive host.

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Beef Tapeworms:

Adult Beef Tapeworms (Taenia saginata) live inside the human intestine, and can reach 20 meters in length (!), though this is exceptional. Three to five meters is more common. Infection comes from eating improperly-cooked beef. (Other species of tapeworms infect pigs and fishes; eating pretty-much any uncooked meat carries a risk of infection.)


Eggs and proglottids produced by adult tapeworms are shed in the host’s feces. If an infected person defecates onto grass, and a cow then eats the contaminated grass, larvae hatch out of the eggs in the cow’s gut and migrate into its muscle tissues. (The larvae cause infected beef to look “measly.”)

If a person eats contaminated beef that has not been cooked to a temperature sufficient to kill the larval tapeworms, they will attach to the intestinal wall. A serious infection will cause the person to lose weight, because the tapeworms are absorbing so many nutrients that would normally be absorbed by the person. There are persistent rumors that some companies sold tapeworm eggs as weight-loss products in the past, though this has never been verified.

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Phylum Mesozoa:

The name Mesozoa comes from the Greek “mesos,” meaning “middle” and “zoon,” meaning “animal.” The name comes from the fact that when they were first described, they were thought to be some sort of “missing link” between the protists and the animals. That’s understandable because mesozoans are tiny, ciliated, wormlike animals; a mesozoan looks like nothing so much as a few dozen ciliate protists clumped together in semblance of a very simple animal.

Most mesozoans live as parasites in marine invertebrates. These creatures are extremely simple animals with no nervous, respiratory, circulatory or digestive systems. In fact, a mesozoan has no organs at all. A typical mesozoan is only a few millimeters long, with only a few dozen cells at most. (Some have only 20 body cells when mature.) Only about 100 species of mesozoans are known.

The mesozoans are a rather mysterious phylum, and no one knows exactly how they should be classified. Some researchers consider them to be degenerate turbellarian flatworms, which is not an unreasonable position. A common observation is that parasites often evolve to become simpler; presumably, this is because they rely on their hosts for many of the bodily functions that a free-living organism would have to perform for itself. And few animals are simpler than mesozoans.

Molecular analyses indicate that mesozoans are indeed lophotrochozoans, but it’s unclear how closely-related they are to the platyhelminths. It’s entirely possible that the Mesozoa are a paraphyletic phylum.

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Phylum Nemertea:

The nemerteans are commonly known as “ribbon worms” or “proboscis worms.” Their proper name comes from the Greek “Nemertes.” Nemertes was one of the Nereids, the “unerring one.” This refers to the fact that a nemertean possesses a long, muscular proboscis that it can thrust out rapidly and with uncanny accuracy. It uses its proboscis to snare prey.

Nemerteans are thread-shaped or ribbon-shaped worms, and there are about 1,000 species. Almost all of them are marine, so they aren’t well known to most people. Nonetheless, some species live in fresh water, and there are a handful of terrestrial species. Most nemerteans are less than 20 centimeters long, though some can reach several meters in length.


The general body plan of a nemertean is very similar to that of a turbellarian flatworm, so they’re generally considered to be acoelomate animals. Like turbellarians, nemerteans have flame cells that are used to expel metabolic wastes, and the epidermis is ciliated.


Of course, there are some important differences between nemerteans and flatworms. For one thing, the nemerteans are the simplest members of the Bilateria to have complete (one-way) digestive systems. Food is taken in through the anterior mouth, and eventually expelled from a different opening at the animal’s posterior end, the anus. A complete digestive system is more efficient, because wastes don’t have to be moved forward and expelled through the mouth. This makes it possible for an animal to ingest food and expel undigestible material at the same time.


Another important difference between nemerteans and flatworms is that the nermerteans are the simplest animals with any sort of circulatory system. Blood circulates through two or three blood vessels that extend down the length of the body, and the blood helps to distribute oxygen and digested material to body tissues. There is no heart, but the walls of the blood vessels are muscular and can contract. This, along with pressure generated by contraction of body muscles move blood through the vessels. Because there is no heart and because the blood vessels have no one-way valves, the flow of blood in a nemertean’s blood vessels frequently reverses.


The truly distinctive feature of a nemertean, though, is its proboscis. The proboscis is normally encased in a sheath just above a nemertean’s mouth, and is surrounded by a fluid-filled cavity known as the rhynchocoel. Since the rhynchocoel is entirely surrounded by mesodermal tissue, it is a true coelom. However, since the rhynchocoel is present only in the anterior portion of a nemertean’s body, and because it seems to have evolved independently of the coelom in eucoelomate animals, nemerteans are usually classified as acoelomates.

Contraction of muscles surrounding the proboscis sheath creates pressure that causes the proboscis to invert, and to shoot out and forward. Most nemerteans use their proboscis to capture smaller animals as prey. The proboscis is coated in a sticky slime and coils around the prey, so prey animals rarely escape. To make matters worse for the prey, the proboscis of most nemerteans is tipped with a barbed stylet. Not only does the stylet pierce and hold prey, but in many nemerteans, it secretes a potent neurotoxin. Once prey is immobilized, the nemertean retracts its proboscis and the unfortunate victim is swallowed whole.

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The flatworms, mesozoans and ribbon worms are generally considered to be the simplest of the Bilateria. That doesn’t mean they aren’t successful, however; the platyhelminths, in particular, are a large and diverse group of animals.

There are some lophotrochozoan phyla that are much larger and more diverse than are the platyhelminths, however. In particular, the phyla Mollusca and Annelida are very large and diverse animal taxa, and everyone is familiar with representatives of these phyla. We’ll discuss those phyla shortly, but first we’ll discuss some of the smaller and less-familiar lophotrochozoan phyla.

Re: An Introduction to Zoology: Chapter 11
 

:shudder: What are the symptoms of a liver fluke infection? I like sushi and has eaten plenty in my life. They were cooked though, or so I thought.

Re: An Introduction to Zoology: Chapter 11
 

Wow! I haven't even finished editing the article into its final form yet!

If you had a liver fluke infection, you'd probably know it. Symptoms include pain in the abdomen, fever, vomiting, and diarrhea.

Cheers,

Michael