360 



MOLLUSCA. 



power, for lesions are repaired by a cement-like substance secreted 



by the mantle. 



The shell consists of one piece, except in the Chitonidae (Fig. 



305) in which there are eight pieces, and it usually has the same 



shape as the visceral sac. When that is flat or conical, the shell 



is also flat and conical (Patella, 

 Fissurella) ; when the visceral sac 

 is spirally coiled, the shell also is 

 spirafty coiled ; and when the mantle- 

 fold has a slit extending along the 

 roof of the mantle-cavity, the shell 

 has a corresponding slit or perforation 

 (Fissurella, Haliotis}. Finally, pro- 

 longations of the edge of the mantle, 

 such as the siphon, leave their mark 

 upon the lip of the shell -aperture 

 (Sipli onostomata). 



The visceral sac of Chiton is not 

 prominent and never coiled. The 

 shell-pieces of this animal are par- 

 tially covered by upgrowths of the 

 mantle, and may be entirely covered 

 by them (Cryptochiton). The shell- 

 beds so formed have been compared 

 by Lankester to the persisting shell- 

 sac (in this case multiple) of the larva. 



The visceral sac and shell are almost 

 always coiled to a certain extent in the 

 young or larva, even if not in the adult. 

 The coils may be lost by decollation, as 

 in Coecam, in which the spiral part drops 

 off; or by modification during growth as 

 in Fissurella (Fig. 281, F, Gf, H), or by 

 subsequent addition during growth (by 

 secretion from two reflected lobes of the 

 mantle) of calcareous matter, which over- 

 lies the spiral shell and hides it, as in 

 the cowries (Cypraea). 



The spiral is generally dextral (leiotropic), i.e., if the shell is placed with 

 its spire uppermost and the aperture towards the observer, the aperture is to 

 the right ; or it may be sinistral. When it is sinistral the organization is 

 sometimes affected, and the organs and apertures usually placed to the right 

 are on the left side. This is always the case with sinistral monstrosities, 

 but not always when, the species is normally sinistral. In Spirialis, Limacina, 



FIG. 281. Shells of A Pleurotomaria, 

 B Polytremaria, C and E Emarginula, 

 D Haliotis, F Fissurella, G and H 

 stages in the growth of the shell of 

 Fissurella (from Lang). 



