MOLLUSOA 167 



of variation in the size and shape of the fool among 

 the various species of gastropods. 



As contrasted to the bivalves, the gastropods 



are often called univalves because they usually pos- 

 sess a single shell, generally coiled or spiral; a few, 



however, such as the slugs and nudibranchs possess 

 no external shell whatever. (These are not treated 

 in this book.) 



Another common name for this group of mol- 

 lusks is Sea Snails. 



The opening through which the animal projects 

 out of the shell is known as the aperture and is in 

 some species covered by a horny or calcareous lid 

 known as the operculum. 



The top of the shell is called the apex and each 

 turn of the spiral is known as a whorl. The sculpt- 

 ure of the whorls is often very complicated and is 

 of importance in the determination of the various 

 species. The outer edge of the aperture is known 

 as the outer lip, while the inner lip is termed the 

 columella. 



Although externally very different from the bi- 

 valves, the internal anatomy of both bivalves and 

 univalves is essentially similar. One structure, how- 

 ever, peculiar to univalves is the radula or tongue. 

 This structure is equipped with many small teeth 

 and is used to grind the food obtained by the mol- 

 lusk. The radula is hidden among the muscular 

 tissue which lines the inside of the snail. The struct 

 ure of the radula is of great importance in the differ- 

 entiation of species, particularly of land mollusks. 



As is the case with the Pelecypoda many gastro- 

 pods are of economic importance as food, although 

 they seem not to be especially favored along our 

 coast. Among others, the Moon Snail (Polinices 



