Arthropods, molluscs, echinoderms and hemichordates

Arthropoda (jointed legs) is the largest phylum: over two-thirds of all named species, including insects. They have a hard chitin exoskeleton, a body in head, thorax and abdomen, and jointed appendages. They breathe by gills, book gills, book lungs or tracheae (air tubes). Circulation is open. Sense organs include antennae, compound and simple eyes, and balance organs (statocysts). Waste is removed by malpighian tubules. Useful ones: honey bee (Apis), silkworm (Bombyx), lac insect (Laccifer). Disease carriers (vectors): mosquitoes Anopheles, Culex and Aedes. Pest: locust (Locusta). Limulus, the king crab, is a living fossil.

Mollusca, the second largest phylum: soft, unsegmented bodies usually covered by a calcareous shell. The body has a head, a muscular foot and a visceral hump. A soft skin fold, the mantle, covers the hump; the space under it holds feather-like gills. The mouth has a file-like rasping tongue, the radula. Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus.

Echinodermata (spiny skin): a skeleton of calcareous ossicles; all marine. Adults are radial, but their larvae are bilateral. Their special water vascular system works their tube feet for moving, catching food and breathing. They have no excretory system. Asterias (starfish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), Ophiura (brittle star).

Hemichordata: small, worm-like marine animals with a body in three parts, proboscis, collar and trunk. They have a stomochord, which looks a bit like a notochord. They were once counted as chordates but are now a separate non-chordate phylum. Balanoglossus, Saccoglossus.