MOLLUSCA. GASTEROPODA. 153 



the columella there is left a space, extending from the 

 base of the shell a greater or less distance towards the 

 apex ; the opening of this space at the base of the shell is 

 called the umbilicus. When there is a columella the 

 shell is said to be imperforate, when instead there is a 

 space it is perforate; sometimes in the latter case the 

 space becomes filled up with a shelly growth, known as 

 callus. 



Usually the cavity of the gasteropod shell is con- 

 tinuous from the apex to the aperture, but in a few cases 

 partitions are thrown across the earlier parts of the shell 

 (fig. 47), forming chambers which afterwards remain 

 empty. The form of the aperture varies considerably in 

 different genera and is of great importance in classifi- 

 cation ; in shape, it may be circular, oval, elongate, 

 oblong, etc. Its margin is termed the peristome; the 

 outer part of which forms the outer lip (L), the inner 

 (that next the columella) the inner lip. As the 

 gasteropod crawls along, the shell is carried on the dorsal 

 surface of its body with the apex directed backwards; 

 consequently the- part of the aperture farthest from the 

 apex is anterior, the opposite (nearest the apex) is pos- 

 terior. Sometimes, as in Natica, there is no break in the 

 aperture, it is then said to be entire or ho los tomato us ; 

 in others the anterior border is notched or produced into 

 a tube (ac) in which the siphon is placed, these forms 

 are said to be siphonostomatous ; sometimes also there is 

 at the posterior border another canal (pc), in which the 

 excurrent or anal siphon is placed. The lip may be thin 

 and sharp, or thickened. Sometimes it is curved out- 

 wards, and is then said to be reflected ; or it is curved in- 

 wards and is inflected. It may be even, or crenulated, 

 or produced into processes. 



