Bases of classification and non-coelomate phyla

Biology · Kingdom Animalia. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Animals are classified by their body plan. Four questions sort them. Do the cells form true tissues? Is the body radial or bilateral? Are there two germ layers (diploblastic) or three (triploblastic)? Is there a body cavity, and is it lined by mesoderm?

Sponges (Porifera) have no true tissues and are called Parazoa. All other animals are Eumetazoa. Cnidaria are radial and diploblastic. Flatworms (Platyhelminthes) are triploblastic but have no body cavity, so they are acoelomates. Roundworms (Nematoda) have a pseudocoelom, a cavity between mesoderm and endoderm that is not fully lined by mesoderm.

Animals with a true coelom are divided into protostomes and deuterostomes.

Rules to remember

Protostomes: blastopore becomes the mouth. Deuterostomes (echinoderms and chordates): blastopore becomes the anus.
Porifera: water enters by ostia, passes the spongocoel and leaves by the osculum; choanocytes (collar cells) make the current and trap food; skeleton of spicules or spongin. Examples: Sycon, Spongilla.
Cnidaria: gastrovascular cavity with one opening, cnidocytes (stinging cells), polyp and medusa forms. Examples: Hydra, Obelia, jellyfish (Aurelia), sea anemone, corals.
Metagenesis: an asexual polyp generation alternates with a sexual medusa generation, as in Obelia.
Platyhelminthes: flat, bilateral, gut with a mouth but no anus, flame cells for excretion. Examples: Planaria, liver fluke (Fasciola), tapeworm (Taenia).
Nematoda: unsegmented round body pointed at both ends, gut with mouth and anus, separate sexes. Examples: Ascaris, hookworm (Ancylostoma), pinworm (Enterobius).

Common mistake

A pseudocoelom is a real, fluid-filled cavity, so nematodes are not acoelomates. It is called false only because mesoderm does not line it on all sides.

Worked example

A worm from the intestine of a pig is long, round and unsegmented, and pointed at both ends. It has a mouth and an anus, a body cavity not lined by mesoderm, and males and females are separate. Name its phylum.

A worm-shaped animal could be a flatworm (Platyhelminthes), a roundworm (Nematoda) or an annelid.
Flatworms are flat, have no body cavity and no anus, so it is not one of them.
Annelids are segmented with a true coelom; an unsegmented worm with a pseudocoelom and a complete gut is a roundworm.

Answer: Phylum Nematoda (Aschelminthes), a roundworm such as Ascaris.

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Chapter 10: Kingdom Animalia

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