THE ANIMAL KINGDOM 1 3 



numbers of individuals of several, they do not lie near the main line of 

 animal evolution and have contributed nothing to the forms above them. 

 Three main classes are separated by differences in the structure and 

 functions of the foot. 



Class Gastropoda 



The gastropods are the snails. In them, the foot is the familiar 

 ventral surface on which the animal crawls. Most have a single shell 

 from which comes their other name, univalves. Commonly, the shell 

 is coiled; but the Chinaman's cap, patella, has an oval dome. Some are 

 earshaped, suggesting one shell of a bivalve. Some of the land snails, 

 which are air-breathing, like the common garden slug, have no shells 

 or only a minute rudiment. In many, the opening of the spiral shell 

 is closed by an operculum when the animal withdraws itself. 



The enormous number of gastropod species, nearly 50,000, the variety 

 and beauty of the shells, and the ease with which they may be pre- 

 served and displayed, have long made conchology a favorite branch of 

 natural history. 



Class Pelecjrpoda 



The pelecypods or lamellibranchs are the common bivalves, such as 

 oysters and clams. They have no heads, and the foot, as their name 

 indicates, is hatchet-shaped. It functions as a burrowing tool. The 

 mantle, in addition to secreting the two shells, is often prolonged into a 

 siphon which, in the common clam, may be elongated to nearly a foot, 

 and reach the surface of the sand while the rest of the animal remains 

 safely buried. Commonly, the siphon has two tubes, through one of 

 which water is drawn into the mantle cavity for respiration and food, 

 while through the other are ejected the body wastes. 



The pelecypods are, in general, less active than the gastropods; but the 

 fresh-water clams crawl about freely on the muscular foot, and the 

 scallops, or pectens, swim by flapping their shells together. 



This group, also, with some ten thousand species, is sought after by 

 collectors. Pearls, and mother of pearl f-or buttons, are obtained from 

 both marine and fresh-water forms. The ship worm, teredo, which often 

 does great damage to ships and docks, is a pelecypod. 



Class Cephalopoda 



The cephalopods are the cuttlefishes, squids, and devilfish. In them, 

 the mantle is wrapped about the body to form a sac, which is nearly closed 

 except near the head, where openings admit water to the gills in the 

 mantle cavity. The foot is divided into long tentacles which surround 

 the powerful beak. The paired eyes are much like those of vertebrates. 



