GASTROPODA. 



755 



choid," "turbinated," &c. This last form is the one which may 

 be looked upon as most characteristic of the Gastropods, the shell 

 being composed of a number of whorls (fig. 640, wh) passing 

 obliquely round a central axis or 

 " columella " (co), having the em- 

 bryonic shell or " nucleus " at its 

 apex (a), and having the mouth 

 or " aperture " of the shell placed 

 at the extremity of the last and 

 largest of the whorls, termed the 

 " body-whorl" {wh'). The lines 

 or grooves formed by the junc- 

 tion of the whorls are termed the 

 " sutures " (su\ and the whorls 

 above the body-whorl constitute 

 the " spire " (s) of the shell. The 

 axis of the shell (columella), round 

 which the whorls are coiled, is 

 usually solid, when the shell is 

 said to be " imperforate " • but 

 it is sometimes hollow, when the 

 shell is said to be "perforated," 

 and the aperture of the axis near 

 the mouth of the shell is called 

 the "umbilicus." The margin of 

 the "aperture" of the shell is 

 termed the "peristome," or "peri- 

 treme," and is composed of an 

 outer and inner lip, of which the 



former (fig. 640, l) is often expanded or fringed with spines. When 

 these expansions or fringes are periodically formed, the place of the 

 mouth of the shell at different stages of its growth is marked by 

 ridges or rows of spines, which cross the whorls, and are called 

 "varices." In certain groups of the Gastropods (Holostomatd) the 

 aperture of the shell is unbrokenly round or " entire," but in other 

 groups (Siplwnostomata) it is notched, or produced into a canal. 

 Often there are two of these canals, an anterior and a posterior, and 

 the function of these is to protect the respiratory siphons. The 

 animal withdraws into its shell by a retractor muscle, which passes 

 into the foot, or is attached to the operculum ; its scar or impression 

 being placed, in the spiral Univalves, upon the columella. In the 

 great majority of the Univalves the shell is coiled to the left, the 

 " mouth " of the shell being thus on the right-hand side (fig. 640). 

 In such cases the shell is said to be right-handed or " dextral." In 

 other cases, however, the shell is coiled to the right, and the mouth 



Fig. 640. — Longitudinal section of Triton 

 corrngatum. S, Spire ; L, Outer lip of the 

 aperture of the shell ; a, Apex ; wh, The 

 last whorl of the spire ; wh', The body- whorl ; 

 su, Suture ; ca, Anterior canal ; fie. Poste- 

 rior canal. 



