246 ELEMENTARY ZOOLOGY 



Development. Reproduction among the molluscs is 

 always sexual. Multiplication by budding or by the 

 parthenogenetic production of eggs is not known to occur. 

 The eggs are usually laid in a mass held together by a 

 gelatinous substance. In most species the young mollusc 

 on hatching from the egg does not resemble its parent, 

 but is a free-swimming larva called a vcliger. It is 

 provided with cilia for organs of locomotion. It must 

 undergo a radical change in order to reach the adult 

 stage. Thus metamorphosis occurs in this branch as well 

 as among the Arthropods and Echinoderms. In the 

 development of some molluscs, however, there is little or 

 no metamorphosis, the young being hatched in a condi- 

 tion much resembling, except in size, the parent. 



Some of the special characteristics of structure, life- 

 history, and habits of the molluscs will be noted in our 

 consideration of the various kinds. 



Classification. The branch Mollusca is divided into 

 five classes, three of which include the more familiar 

 kinds. These three classes are the Pelecypoda, including 

 the mussels, cockles, clams, scallops, oysters, etc., mol- 

 luscs with a shell composed of two pieces, one on each 

 side of the body and hinged together; the Gastropoda, 

 including the snails, slugs, periwinkles, whelks, and a host 

 of other univalved shell-fish, that is, molluscs which have 

 a shell composed of a single piece ; and the Cephalopoda, 

 including the squids, cuttlefishes, octopi, and the pearly 

 nautilus. 



Clams, scallops, and oysters (Pelecypoda). TECHNICAL 



NOTE. Shells of scallops, oysters, and sea-rnussels should be had for 

 examination ; also specimens of Teredo or Pholas in alcohol or for- 

 malin, and pieces of pile bored by Teredo. Make drawings of vari- 

 ous bivalve shells, and of Teredo. 



The fresh-water mussel which we have studied is an 

 example of the bivalve molluscs. The members of this 



