Other Invertebrate Phyla: From Sponges to Starfish

1Phylum Porifera: sponges

• The simplest animals: many cell types but no true tissues or organs.
• The body is full of pores. Water is drawn in through them, food is filtered out, and the water leaves through a large opening called the osculum.
• Asymmetrical; adults are fixed to rocks (sessile). The body is supported by tiny needles called spicules.
• Almost all live in the sea. Example: the bath sponge.

2Phylum Cnidaria: jellyfish, Hydra, corals and sea anemones

• Radial symmetry and only two cell layers (diploblastic).
• A sac-like gut with one opening, which acts as both mouth and anus.
• A ring of tentacles armed with stinging cells that paralyse prey.
• Two body forms: the polyp (a tube fixed at one end, like Hydra) and the medusa (a free-swimming bell, like a jellyfish).
• Hydra, about 1 cm long, lives in freshwater ponds; the rest are mostly marine.

3Phylum Platyhelminthes: flatworms

• Flat, thin, unsegmented bodies with bilateral symmetry and three cell layers, but no body cavity.
• No circulatory or breathing system: gases simply diffuse through the body surface. A flat body keeps every cell close to the surface.
• A gut with one opening, or no gut at all in tapeworms, which absorb food from their host through their body wall.
• Most are parasites and many are hermaphrodites (one animal has both male and female organs).
• Examples: the free-living planarian; the tapeworms Taenia, caught by eating undercooked pork or beef containing larvae; and the blood fluke Schistosoma, which causes bilharzia (schistosomiasis).

4Phylum Nematoda: roundworms

• Round, thread-like, unsegmented bodies, pointed at both ends, covered by a tough cuticle.
• A complete gut with a mouth and an anus.
• Usually separate sexes.
• Many live freely in soil. Parasitic examples include Ascaris (the large roundworm of the human intestine), hookworms, which enter through the skin of bare feet, and root-knot nematodes that damage crops.

5Phylum Annelida: segmented worms

• The body is divided into ring-like segments, both outside and inside.
• A true body cavity and a closed circulatory system.
• Tiny bristles called chaetae grip the soil during movement; leeches have none, but use suckers instead.
• Moist skin is the breathing surface.
• Examples: earthworms, which are hermaphrodites with a thick band called the clitellum; leeches; and ragworms in the sea.

6Phylum Mollusca: snails, slugs, octopuses and clams

• A soft, unsegmented body with a muscular foot for moving.
• A fold of skin, the mantle, which in many species makes a shell of calcium carbonate.
• Most have a radula, a rough tongue that scrapes food.
• Examples: snails and slugs (pests of vegetables), octopuses and squid, and oysters and mussels. Some water snails are the hosts in which the bilharzia fluke develops.

7Phylum Echinodermata: starfish and sea urchins

• All marine, so none live wild in Rwanda.
• Spiny skin over an internal skeleton of chalky plates.
• Adults have five-part radial symmetry, although their larvae are bilateral.
• A water vascular system works hundreds of small tube feet used for moving and feeding.
• Examples: starfish, sea urchins, sea cucumbers.

Worked example: how bilharzia spreads, and how to stop it

Question: Describe how a person catches bilharzia and suggest three ways to prevent it.

Solution: Adult Schistosoma flukes live in a person's blood vessels. Their eggs leave the body in urine or faeces. If these reach a lake, pond or marsh, the eggs hatch, and the larvae enter certain water snails and multiply. The snails release thousands of tiny larvae into the water, which burrow through the skin of anyone who wades, swims, bathes or washes clothes there.

To prevent it: (1) use latrines, so that eggs never reach the water; (2) avoid wading or bathing in still water known to carry the disease, and use safe piped or boiled water; (3) treat infected people with medicine; (4) reduce snail numbers, for example by clearing water weeds.

Practice