Invertebrates

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Turbellaria (mostly free-living flatworms)

Monogenea (monogeneans)

Trematoda (trematodes, or flukes)

Cestoda (tapeworms)

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Turbellarians

Turbellarians are nearly all free-living and mostly marine.

The best-known turbellarians are commonly called planarians.

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Fig. 33-9 A marine flatworm (class Turbellaria)

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Planarians have light-sensitive eyespots and centralized nerve nets.

The planarian nervous system is more complex and centralized than the nerve nets of cnidarians.

Planarians are hermaphrodites - possess both male and female gonads / sex organs. Planarians can reproduce sexually, or asexually through fission.

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Anatomy of a planarian

Pharynx

Gastrovascular

cavity

Mouth

Eyespots

Ganglia

Ventral nerve cords

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Fig. 33-11 The life cycle of a blood fluke. Like many trematodes, it is a parasite.

Human host

Motile larva

Snail host

Ciliated larva

Male

Female

1 mm

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Tapeworms

Tapeworms are parasites of vertebrates and lack a digestive system.

Tapeworms absorb nutrients from the host’s intestine.

Fertilized eggs, produced by sexual reproduction, leave the host’s body in feces.

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Fig. 33-12 Anatomy of a tapeworm

Proglottids with

reproductive structures

Hooks

Sucker

Scolex

200 µm

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Rotifers

Rotifers, phylum Rotifera, are tiny animals that inhabit fresh water, the ocean, and damp soil.

Rotifers are smaller than many protists but are truly multicellular and have specialized organ systems.

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Fig. 33-13 A rotifer

Jaws

Crown

of cilia

Anus

Stomach

0.1 mm

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Rotifers have an alimentary canal, a digestive tube with a separate mouth and anus that lies within a fluid-filled pseudocoelom

Rotifers reproduce by parthenogenesis, in which females produce offspring from unfertilized eggs.

Some species are unusual in that they lack males entirely

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Lophophorates: Ectoprocts and Brachiopods