THE GROUPS OF ANIMALS 



271 



the clams and mussels (Fig. 231) the foot is a wedge which plows through 

 the sand or mud. In the snails (Fig. 232) it is flat, and the animal creeps 

 along on it, usually by rapid wavelike muscular contractions, but some- 

 times by means of cilia. The foot of a snail may secrete a mucous 

 substance along which the animal creeps; a vertical 

 roadway may thus be erected directly through the water 

 without any support except at one end. The foot of the 

 squids (Fig. 233), cuttlefishes, and nautili is transformed 

 into a circle of arms bearing suckers. 



The shell consists of two valves in the clams, oysters, 

 and mussels; is spirally wound in the snails; is a row of 

 movable plates in the chitons; is entirely embedded in the 

 flesh in the cuttlefishes, squids, and most slugs; and is 

 entirely lacking in certain marine mollusks called nudi- 

 branchs, which bear some resemblance to snails, and in a 

 few species of several other groups. 



The sexes are usually separate, but one class of snails is hermaphro- 

 ditic, as are also some members of other classes. Among the latter, self- 

 fertilization may occur, or two animals may mate. 



The mollusks began as marine animals, then began to invade fresh 

 water, and finally the land. Only the snails have gone on land, however. 



Fig. 230. 

 — Chiton. 

 (.Courtesy of 

 Carolina Bio- 

 logical Supply 

 Co.) 



Fig. 231. — A clam. 



Fig. 232. — A snail. 



Fig. 233.— a squid. 



and only the snails and clams into fresh water. The group as a whole 

 has always been a successful one. It has maintained its abundance 

 throughout geological time and is as well represented by number of species 

 now as it ever has been. The evolution of mollusks has in general led to 



a reduction of the shell and the growth of the 

 mantle over it, though one class has escaped 

 these changes. 



Man has an economic interest in mollusks 

 in several ways. Oysters and clams are im- 

 portant articles of food, the former being 

 extensively cultivated. Pearls are made up of layers of nacre secreted 

 around some irritating foreign object by the epithelial cells of clams. 

 Such objects are deliberately inserted by pearl raisers, and pearls of any 

 desired shape may be obtained. The same substance, nacre, on the 

 inside of clam shells constitutes mother of pearl, which is used for buttons 



Fig. 234.— Teredo, the 

 ship worm. {Courtesy of 

 Carolina Biological Supply 

 Co.) 



