SHELLS OF MOLLUSCS 



469 



probable that the mantle has much importance in respira- 

 tion, the gills being perhaps of most importance in connec- 

 tion with nutrition, and as brood-chambers. In those 

 Gasteropods in which the gills are suppressed, there are 

 often special respiratory organs (" adaptive gills "), such as 

 the circle of plumes around the anus in Doris and its allies 

 (Fig. 266). The osphradia are absent in Cephalopods, 

 except in Nautilus, and one at least is usually suppressed in 

 Gasteropods. 



Shell. — On the dorsal surface of almost every mollusc 

 embryo there is a little shell- 

 sac in which an embryonic 

 shell is begun ; the adult shell, 

 however, is always started and 

 increased by the mantle. Like 

 other cuticular products, it has 

 an organic basis (conchiolin or 

 conchin), along with which 

 carbonate of lime is associ- 

 ated. There is a thin outer 

 '* horny " layer, a thick median 

 " prismatic " stratum of hme, 

 and an internal mother-of-pearl 

 layer, which may be divided 

 into two strata by a clear 

 intermediate layer, well seen 

 in the fresh-water mussel, 

 Margaritana margaritifera. 



Pearls are formed in sacs of the 

 external epithelium of the mantle, 



sometimes aromid a centre of a periostracum-like substance, sometimes 

 around the larva of a Trematode or Cestode. They are to be dis- 

 tinguished from concretions formed around an intruded irritant particle. 

 The latter do not show the characteristic lamination of pearls. Some 

 pearl-like structures are fixed to the shell ; true pearls are free. While 

 some investigators insist on the parasitic origin of pearls, others are 

 equally emphatic in declaring that they may arise independently. But 

 all are agreed that they are pathological products. 



Larvae. — In their life-history m-ost IMolluscs pass 

 through two larval stages. The first of these is a pear- 

 shaped or barrel-shaped form, with a curved gut, and with 

 a ring of cilia in front of the mouth. It is a " trocho- 



FiG. 267. — Ventral surface of 

 Patella vulgata. — After Forbes 

 and Hanley. 



Note simple eyes at base of tentacles, 

 mouth, median foot, and vascular 

 margin of mantle replacing the 

 absent giUs. 



