THE SHELL. 45 



columella, however, is usually applied to that portion of the 

 inner wall of the shell which invests the axis — whether it be 

 solid or open : it therefore is the lower portion of the inner 

 margin of the aperture in most spiral shells. It is wanting in 

 discoidal shells (Planorbis), hidden or minute in convolute shells 

 (Bulla and Cj^praaiand in ear-shaped shells (Haliotis). Non- 

 spiral shells have no columella. 



The aj)ex or posterior end of the shell presents important 

 characters, as it was the nucleus or part formed in the egg ; it 

 is sinistral in the Pja-amidellidfe, oblique and spiral in the 

 Nucleobranchs and Emarginnlse, and mammillated in Turhinella 

 pymnn and Fusas antiquus. 



The apex is directed backwards in all except some of the 

 Patellidffi, in which it is turned forwards, over the animal's head. 

 In the adult condition of some shells the apex is always truncated 

 (or decollated) as in Cylindrella and Bidinms decollatus; in others 

 it is only truncated when the animals have lived in acidulous 

 waters ( e. g. Cerithidea and Pirena;, and specimens may be 

 obtained from more favorable situations with the points perfect. 



A tohorl measures a single complete revolution of the spiral 

 cone ; its peyHphery is an imaginary spiral upon the outer wall, 

 half-way between the suture and base : line of greatest width. 

 In counting the number of whorls in a shell commence with the 

 aperture-margin, whence, to a point on the suture of the volution 

 above it constitutes one whorl ; repeat this process to the apex. 



The line of channel formed by the junction of the whorls is 

 .termed the suture. 



The last turn of the shell ending with the aperture (body-ivhorl) ^ 

 is usually very capacious ; the others are collectively called spire- 

 whorls. In the females of some species the whorls enlarge more 

 rapidl}^ than in the males (e, g. Buccinum undatum). The base 

 or anterior extremity of the shell is the opposite end to the apex, 

 and is usually the front of the aperture. 



The distance between the apex and base of a gastropod shell 

 is termed its height. It has usually been called lengthy but the 

 other designation corresponds better with the terminology of 

 the lamellibranchiates as well as the non-spiral gastropods : thus 

 in Patella, the distance from summit to base is height, from 

 anterior to posterior margin is length, from side to side is breadth. 

 In spiral shells the breadth or diameter is the distance across 

 the body-whorl at its periphery. 



The aperture is entire in most of the vegetable feeders (holos- 

 tomata), but notched or produced info a canal, in the carnivorous 

 families ( siphonostomata) ; this canal surrounds a siphon, which 

 is respiratory in its office, and does not necessarily indicate the 

 nature of the food. Sometimes there is a posterior channel or 

 canal, which is excurrent, or anal, in its function ( e. g. Strombidse 



