SHELLS. 9 



For convenience sake, therefore, mollusks are 

 described in the following pages chiefly by their 

 shells ; but the student is earnestly advised, when- 

 ever it is possible, to carefully study the organs 

 and habits of the living animals. 



The parts of the shell may easily be learned 

 by referring to the diagrams on Plate I. The 

 univalve shell, Fig. 1, consists of a single tube, 

 coiled in a spiral manner round a central axis. 

 This tube grows larger as it advances, and 

 usually leaves marks which indicate its stages, 

 which are called lines of growth. The tube 

 varies much in form in different species, some- 

 times being flattened, sometimes angled, and 

 again, nearly cylindrical. The opening of the 

 tube, which often may be closed by the opercu- 

 lum, is called the aperture, and this also varies 

 greatly in form, being sometimes nearly round, 

 and in other species prolonged into a tube or 

 trough, called the canal. Those mollusks whose 

 shells have this canal, (ca, Fig. 1 ), are mostly 

 carnivorous, while most round-mouthed shells 

 belong to vegetable feeders. Each complete 

 turn of the spiral tube is called a whorl, the last 

 and largest being the hody whorl, and the others 

 forming the spire. The point of beginning is 

 called the apex, and the spiral line separating the 

 whorls is the suture. The axis round which the 

 tube revolves is called the columella; sometimes 

 it is hollow for a little way, then this indentation 

 is known as the umbilicus. The aperture, some- 

 times called the mouth, is hounded on one side 

 by the outer lip / the inner lip is commonly 

 grown to the body whorl or the columella. Spi- 



