Cartilaginous fishes such as sharks have to keep swimming because they lack

The correct answer is B, Arthropoda. More than three-fourths of all described animal species are arthropods, that is insects, spiders, scorpions, centipedes, crabs and prawns. A hard chitinous exoskeleton, jointed appendages, small body size and, in insects, the power of flight allowed the group to spread into almost every habitat on earth.

Option A, Mollusca, is the second largest phylum, with snails, mussels, squids and octopuses, and is the usual wrong answer here. Option C, Chordata, contains all the vertebrates and seems large because the familiar animals belong to it, but it has far fewer species than either Arthropoda or Mollusca. Option D, Annelida, the segmented worms, is a much smaller phylum of earthworms, leeches and marine worms. The order Arthropoda, Mollusca and then Chordata is worth remembering.

The correct answer is C, Echinodermata. Starfishes, sea urchins, sea cucumbers, brittle stars and sea lilies are found only in the sea, and their water vascular system, which works by taking in sea water, is one reason they never colonised fresh water or land. The phylum Ctenophora, the comb jellies, is also exclusively marine.

Option A, Annelida, includes the earthworm on land and the leech in fresh water besides marine worms such as Nereis. Option B, Mollusca, includes the land snail, the freshwater Pila and marine squids and octopuses. Option D, Platyhelminthes, includes the freshwater Planaria and parasites that live inside the bodies of land animals, such as the tapeworm. Only the echinoderms, with the comb jellies, are confined to the sea. A further mark of the phylum is that its members are radially symmetrical as adults but bilaterally symmetrical as larvae.

The correct answer is A, flatworms, the phylum Platyhelminthes. Flame cells, also called solenocytes, are specialised cells with a tuft of beating cilia that drives waste fluid into a system of tubules, and they handle both the removal of nitrogenous waste and the balance of water in animals such as Planaria, Taenia and Fasciola.

Option B, roundworms of Aschelminthes, excrete through a simple excretory pore and renette cells, not flame cells. Option C, earthworms and the other annelids, excrete through nephridia, a pair of coiled tubules in nearly every segment. Option D, insects, excrete through Malpighian tubules opening into the gut, which is how they lose very little water. Matching the excretory structure with the phylum is a standard one-mark question, so learn the four names together: flame cells, renette cells, nephridia and Malpighian tubules, in that order of phyla.

The correct answer is B, Porifera. In sponges the cells are arranged loosely and work with a good deal of independence, with collar cells driving water through a canal system and other cells digesting the food particles, so the body never rises to the level of a true tissue. The skeleton is of spicules or spongin fibres.

Option A, Coelenterata, has reached the tissue level, with stinging cnidoblasts and contractile cells acting together. Option C, Ctenophora, also has the tissue level, with its eight rows of comb plates beating in order. Option D, Platyhelminthes, has gone further still, to the organ level, with definite reproductive and excretory organs. The ladder to remember is cellular, tissue, organ and organ-system, in that order of phyla, and the sponge is also the only one of the four that is usually asymmetrical in body form.

The correct answer is A, Coelenterata, also called Cnidaria, and the phylum takes its name from these cells. Each cnidoblast holds a capsule called a nematocyst with a coiled thread that shoots out and injects a toxin when touched, and it is found in Hydra, the sea anemone, the jellyfish and the corals.

Option B, Echinodermata, defends itself with spines and with pincer-like structures, not with stinging cells. Option C, Mollusca, has a shell and a rasping radula, and in the cone snails a venomous dart, but no cnidoblasts. Option D, Porifera, has spicules that make the body gritty and unpleasant to eat. The presence of cnidoblasts, radial symmetry and a diploblastic body are the three marks of the coelenterates, and the phylum also shows two body forms, the fixed polyp and the free-swimming medusa.