GENERAL NOTES ON MOLLUSCS. 



415 



the common limpet (Patella) (Fig. 225), as well as in 

 terrestrial forms like the snail, where the mantle cavity 

 forms the pulmonary chamber. Even in Lamellibranchs, 

 where the gills are present in much modified form, it is 

 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. 224). The osphradia are 

 absent in Cephalopods, except 

 in Nautilus^ and one at least 

 is usually suppressed in Gas- 

 teropods. 



Shell On the dorsal surface 

 of almost every mollusc em- 

 bryo there is a little shell-sac 

 in which an embryonic shell is 

 begun; the adult shell, how- 

 ever, is always started and 

 increased by the mantle. Like 

 other cuticular products, it has 

 an organic basis (conchiolin or 



conchin), along with which FlG - 225. Ventral surface of 

 carbonate of lime is associated. 



" ^^ 



There is a thin outer " horny Note simple eye ' s at base of tentadeSj 



- *^ ~A;^ * A ,,oo^,,io^ 



mouth, median foot, and vascular 

 margin of mantle replacing the 

 absent gills. 



layer, a thick median pris- 



matic" stratum of lime, 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. 



Mr. Irvine's experiments at Granton Marine Station suggest that the 

 lime salt originally absorbed is not the carbonate (of which there is a 

 scant supply in sea-water), but the sulphate (which is abundant), and 

 that the internal transformation from sulphate to carbonate is perhaps 

 associated with the diffuse decomposition of nitrogenous waste products. 

 Thus carbonate of ammonia, which seems to occur abundantly in the 

 mantle of perfectly fresh mussels, would, with calcium sulphate, yield 

 carbonate of lime and ammonium sulphate. One cannot suppose that 

 shell-making is expressible in a chemical reaction of this simplicity, but it 

 28 



