Biology 2nd Paper (Zoology) · Classes 11-12 · Chapter 1

Coelom, Notochord and Alimentary Canal.

Is every empty space inside a body a coelom? No. A cavity earns that name only with a particular origin and a particular lining. Check those two things and you can tell which group an animal belongs to.

1. What a coelom is

As a triploblast embryo develops, a cavity forms out of the mesoderm layer. If that cavity is wrapped in a membrane called the peritoneum, built of mesodermal cells, it is a coelom. Put simply, the coelom is the space between the alimentary canal and the body wall, lined by mesodermal peritoneum.

So not every cavity inside the body is a coelom; other kinds of cavity lie in there too. Two questions are enough: did the cavity come from the mesoderm, and does a peritoneum surround it?

2. The three groups

By whether a coelom is present, and what kind, animals fall into three groups: acoelomate, pseudocoelomate and eucoelomate.

Acoelomates have no coelom. The embryonic space, the blastocoel, is packed with mesodermal spongy parenchyma cells. The phyla here are Porifera, Cnidaria, Ctenophora and Platyhelminthes, such as the tapeworm (Taenia).

Pseudocoelomates, the false coelomates, also lack a true coelom. A layer of mesodermal cells sometimes lies around the blastocoel, but the cells never form a complete layer or peritoneum that wraps it fully. The phyla are Nematoda, Rotifera and Kinorhyncha; the roundworm Ascaris lumbricoides is one.

In eucoelomates, the true coelomates, the coelom arises as a cavity inside the embryonic mesoderm and is wholly enclosed by a peritoneum of flattened mesodermal epithelial cells. The phyla are Mollusca, Annelida, Arthropoda, Echinodermata, Hemichordata and Chordata, and they are regarded as relatively advanced. Earthworms, cockroaches, snails, frogs and humans all have a coelom.

The difference between pseudocoelomates and eucoelomates fits on one line: in pseudocoelomates the mesodermal cells do not fully surround the blastocoel, in eucoelomates the peritoneum fully surrounds the coelom.

Switch between the three cross-sections and light one layer at a time. Where the mesoderm lies is the whole story.

Sort twelve phyla into three boxes. One question decides it: is there a peritoneum, and does it enclose fully?

3. In a drawing and in a table

Figure 1.9 shows cross-sections of three animals. Parenchyma, pseudocoel and coelom look much alike in a drawing, so pick up a label and tap the right mark. Remember that the peritoneum appears only in the earthworm figure.

The book sets a task: compare pseudocoelomates and eucoelomates. Tap any cell and the comparison for that row appears below.

Now check a cavity along a two-question path: did it come from the embryonic mesoderm, and is it fully lined by peritoneum? At the end see where the earthworm, Ascaris and Taenia land.

4. The notochord

Along the dorsal midline of the body, in the embryo or throughout life, lies a somewhat flexible, elastic rod of tissue. It is the notochord.

The presence or absence of a notochord splits the animal kingdom in two: nonchordates and chordates. Nonchordates never have a notochord, for example the earthworm and the grasshopper. Chordates have it throughout life or only in the embryo, for example Ascidia, the frog, the snake and the human. Even an embryo-only notochord makes an animal a chordate.

5. The alimentary canal

Animals are also split in two by whether an alimentary canal is absent or present: Parazoa and Enterozoa.

Parazoa have no definite alimentary canal or cavity in the body; an example is phylum Porifera. Enterozoa have a definite alimentary canal or cavity, and every animal from Cnidaria to Chordata is an example.

6. Check yourself

The first question is about the coelom groups.

In the second, keep the peritoneum in mind.

The last turns on the notochord or the canal.

The whole note on six cards.

Source: Biology 2nd Paper (Zoology), Classes 11-12 (NCTB), Chapter 1 “Animal Diversity and Classification”, the “Coelom, Notochord and Alimentary Canal” part.