GASTEROPODA I STROMBID.E. 477 



phosis ; the latter at first have a shell capable of entirely 

 concealing them, and closed by an operculum, and, in- 

 stead of creeping, they swim with a pair of ciliated ap- 

 pendages growing from the sides of the head. The shells 

 of Gasteropods, though always single, and usually spiral, 

 vary almost endlessly in form and color. The names 

 applied to the different parts of a gasteropod shell are 

 shown by Fig. 381. Shells which are always concealed 

 by the mantle are colorless ; those which are covered 

 when the animals expand are glazed or enamelled on the 



Fig. 381. 



Spire. W, whorls. 



Columella. 



Umbilicus .... >^5irY a"""* 



Aperture. 



Canal 



surface ; and all other shells are more or less covered 

 with an epidermis. The Class is composed of three 

 orders, Gasteropoda proper, Heteropoda, and Ptero- 

 poda. 



SUB-SECTION I. 



THE ORDER OF GASTEROPODA PROPER. 



THIS Order embraces about forty families, or, accord- 

 ing to the more recent writers, a much greater number, 

 as several of the old families have been much subdivided. 



STROMBID^E, OR STROMBUS FAMILY. This Family 



