GASTEEOPODA. 



205 



but more commonly the inner side of the spiral tube is formed 

 by the pre-existing whorls (Fig. 69). 



The axis of the shell, around which the whorls nre coiled, is 

 sometimes open or hollow ; in which case the shell is said to be 

 perforated, or umbilicated (e.g. solarium). The perforation may 

 be a mere chink, or fissure {rima), as in lacuna; or it may bo 

 filled up by a shelly deposit, as in many naticas. In other shells, 

 like the triton, the whorls are closely coiled, leaving only a 

 pillar of shell, or columella, in the centre ; such shells are said 

 to be imperforate. 



\ apex. 



/ posterior canal. 



/ anterior canal. 



Section of a spiral univalve.* 



The apex of the shell presents important characters, as it 

 was the nucleus or part formed in the egg ; it is sinistral in 

 the pyramidellidce, oblique and spiral in the nucleobranchs and 

 emarginulce, and mammillated in Turlinella pyrum and Fusua 

 antiquus. 



The apex is directed backwards in all except some of the 

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



* Fig. 69. Longitudinal section of Triton corrugatiis, Lam,, from a specimen in the 

 cabinet of Mr. Gray. The upper part of the spire has been partitioned off many times 

 successively. 



