Animal Kingdom: Invertebrates and Vertebrates Notes

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.

The correct answer is B, the octopus. It belongs to the phylum Mollusca and the class Cephalopoda, so it is a relative of the squid and the cuttlefish rather than of any fish, and the common name devil fish came from its appearance and its eight arms with suckers.

Option A, the star fish, is an echinoderm, again not a fish, but it is called the sea star rather than the devil fish. Option C, the sea horse, Hippocampus, is a genuine bony fish of the class Osteichthyes, remarkable because the male carries the eggs in a brood pouch. Option D, the flying fish, Exocoetus, is also a true bony fish, which glides above the water on enlarged pectoral fins. Questions of this kind test whether a candidate separates common names from real classification.

The correct answer is B, Malpighian tubules. These are fine blind tubules that float in the body fluid of an insect and open into the gut, where they take up nitrogenous waste and pass it out as nearly dry uric acid, so that very little water is lost. This is one reason insects survive in dry habitats.

Option A, nephridia, are the excretory organs of annelids such as the earthworm. Option C, flame cells, belong to the flatworms. Option D, green glands, also called antennal glands, are the excretory organs of crustaceans such as the prawn, so they belong to the same phylum as insects but to a different class, which makes them the most tempting wrong answer. Arthropoda is therefore a phylum with more than one excretory plan.

The correct answer is B, an air bladder. Bony fishes of the class Osteichthyes fill or empty a gas-filled air bladder and so hover at any depth without effort. A shark, of the class Chondrichthyes, has no such bladder, and since its body is denser than sea water it sinks unless it keeps moving, using the lift from its stiff pectoral fins and the buoyancy of its large oily liver.

Option A is wrong because sharks have gills, in five to seven pairs of gill slits without an operculum. Option C is wrong because a shark has a backbone of cartilage and is a true vertebrate. Option D is wrong because all fishes, cartilaginous and bony alike, have a two-chambered heart with one atrium and one ventricle; having one is normal and is not a handicap.

The correct answer is B, the crocodile. Reptiles in general have a three-chambered heart, with two atria and a ventricle that is only partly divided, so that oxygenated and deoxygenated blood mix to some extent. In crocodiles and alligators the ventricle is completely divided, giving a four-chambered heart like that of birds and mammals, which suits an animal that holds its breath while submerged.

Option A, the turtle, option C, the cobra, and option D, the chameleon, all have the usual reptilian three-chambered heart. The crocodile exception is one of the most frequently asked facts from this chapter, and it is often paired in statement questions with the other exceptions, that the platypus lays eggs although it is a mammal and that the whale is a mammal although it lives in the sea.

The correct answer is B, the platypus, Ornithorhynchus, of Australia. It belongs to the monotremes, the only mammals that lay eggs, the other being the spiny anteater or echidna. The young hatch from the eggs and are then fed on milk from the mother's mammary glands, which is why the animal is still a mammal.

Option A, the kangaroo, Macropus, is a marsupial: its young are born very small and complete their growth in a pouch, but they are born, not hatched. Option C, the bat, Pteropus and others, is viviparous and is the only mammal capable of true flight. Option D, the whale, is also viviparous and suckles its calf under water; it is a mammal and not a fish. Only the monotremes break the rule about giving birth to young.

The correct answer is B, Cephalochordata. In animals such as Branchiostoma, also called Amphioxus or the lancelet, the notochord extends from the head to the tail and remains there for the whole of life, which is why the group is treated as the clearest surviving picture of the basic chordate plan.

Option A, Urochordata, includes Ascidia, Salpa and Doliolum, in which the notochord is present only in the tail of the free-swimming larva and is lost when the animal settles down, a change called retrogressive metamorphosis. Option C, Vertebrata, replaces the notochord during development with a vertebral column of cartilage or bone. Option D, Hemichordata, is now treated as a separate phylum of non-chordates and has only a short structure in the collar region, not a true notochord.

The correct answer is A, the leech, Hirudinaria, an annelid that feeds on the blood of other animals. Its salivary glands secrete hirudin, an anticoagulant that keeps the host's blood flowing while the leech feeds, and the same property made leeches a part of old medical practice and a subject of modern research on blood thinners.

Option B, the earthworm, Pheretima, feeds on decaying organic matter in soil and secretes no such substance; its service is to soil fertility, which is why Darwin called earthworms the friends of the farmer. Option C, the tapeworm, Taenia, is a flatworm that absorbs digested food through its body wall in the host's intestine. Option D, the roundworm, Ascaris, is also an intestinal parasite. Only the leech is a blood feeder with an anticoagulant.