CLASSES OF ANIMALS 453 



oysters, mussels, scallops, periwinkles, and abalone. The shells 

 of many of the bivalves (pelecypods) are used for making 

 buttons, knife handles, and other small wares. The mother-of- 

 pearl, or nacre, from the inner face of many shells, is used for 

 ornamental purposes. Crushed shells are used to supply lime 

 for chickens. The cuttlefish discharges a dark fluid into the 

 w^ater when startled or pursued ; this is the source of sepia, a 

 brown pigment used by artists. In former times a mollusk sup- 

 plied, the highly prized Tyrian purple, a deep-red pigment formed 

 by the oxidation of some of the animal's juices. Squids and 

 cuttlefish are used by fishermen as bait. It is from a bivalve 

 that pearls are obtained (see section 330). On the whole, the 

 mollusks are probably the most valuable of the invertebrate 

 animals, but they are not all of value (see Fig. 4). The oyster 

 drill is a small, snail-like animal that bores through the oyster 

 shell and does considerable damage. Many of the land snails and 

 slugs become serious pests, destroying garden and truck plants. 



335. Vertebrates. The branch of animal life which includes 

 the backboned animals has also a few rather rare forms that are 

 not, strictly speaking, vertebrates, yet have many resemblances 

 to them. Of these lower forms the lampreys and hagfish are im- 

 portant because they are sometimes parasitic on useful food 

 fish, and the lancelet has been used to some extent as food in 

 tropical islands. Outside of these lower forms, all the animals 

 in this great division are arranged in five classes (see page 86). 

 All these animals are of course a part of the general scavenger 

 and food cycle. In each class we shall find large contributions 

 to our food supply, fish, birds, and mammals being most impor- 

 tant in this respect. In each class we shall find also animals 

 that are of direct use to us in some way, the mammals and birds 

 leading the others; and in each class we shall find some real 

 enem.ies, the mammals again leading. Because of their impor- 

 tance we shall consider each class separately. 



336. Fishes. The general form of the body shows adaptation 

 to movement in the water; yet there are great differences in 

 actual shape, from the nearly cylindrical eel to the flat skate 



