OR OSSEOUS SYSTEM. 47 



or have the valves very unlike each other. The waved 

 disposition of the calcareous particles in the layers of 

 these shells, often gives them a beautiful nacreous lustre, 

 especially in the more pellucid internal layers, which 

 constitutes the mother-of-pearl; and when spherical glo- 

 bules of this matter, composed of concentric layers en- 

 veloping some extraneous particle, are secreted by the 

 mantle and unconnected with the shell, they form the rich 

 pearls of commerce. The edge of the mantle sometimes 

 extends itself beyond the margin of the valves, so as to 

 touch some extraneous body, and by their exuding the 

 usual calcareous and albuminous matter, it causes the con- 

 tiguous surfaces to become cemented together ; in this 

 manner the oyster glues its shell immovably to the rocks at 

 the bottom of the sea. 



XVI. Gasteropoda. The shells of the gasteropods are 

 composed, like other molluscous shells, of carbonate of lime 

 with animal matter. They are extravascular, and generally ex- 

 creted from the outer surface of the skin in the form of hol- 

 low unilocular laminated cones, which envelope the exterior 

 of the animal, and they grow by the addition of successive 

 layers to their inner surface. They take the form of the exterior 

 of the body, especially of the mantle, and they are perma- 

 nently connected with the muscular system. The laminae 

 which compose these shells have generally a fibrous struc- 

 ture, and the parallel fibres which form them have a zig- 

 zag direction nearly vertical to the surfaces of the layers. 

 They are generally covered with a thin epidermic varnish, 

 which protects the colouring matter of the surface ; their 

 outer layers are commonly more loose, opaque and porous 

 than the dense, pellucid, glassy layers of the interior, and 

 they are often provided with an operculum which is some- 

 times horny, sometimes calcareous, and is permanently 

 attached to the muscular foot which secretes it. Many 

 gasteropods have no shell, as the scyllaea, the tritonia, and 

 the doris ; some have a thin calcareous lamina within the 

 skin of the back, as the aplysia, and some have an ex- 

 ternal shell so small as to cover only a portion of the 

 animal's surface, as the testacella, the cryptostoma and the 

 carinaria. The shell is perforated in the haliotis the fissu- 

 rella and the emarginula, and it is composed of eight trans- 



