396 PHYLUM MOLLUSC A. 



in other Mollusca show much variation when compared 

 with this primitive type. Thus the gills may be totally 

 suppressed and the mantle may directly take on a respir- 

 atory function. This occurs in many marine Gasteropods, 

 for example, in the common limpet (Patella) (Fig. 197), 

 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 

 respiration, the gills being perhaps of most importance in 

 connection 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. 196). The osphradia are absent in Cephalopods, 

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

 Gasteropods. 



Shell. Mollusc shells are very beautiful, alike in form 

 and colour. They grow larger by month and year, and 

 mark their progress by rings of growth and changing tints. 

 That they afford their bearers efficient protection, is shown 

 by the appreciation which some hermit crabs exhibit for 

 stolen whelk or buckie shells. More precise observation 

 shows us that the shell consists in great part of carbonate 

 of lime; that it has a thin outer "horny" layer, a thick 

 median "prismatic" stratum of lime, and an internal mother- 

 of-pearl layer. 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, begins 

 on a separate area of the skin, and it is always lined and 

 increased by the mantle. If the increase of the shell be 

 carfully watched in young Molluscs, or if chemical analysis 

 be made, it becomes plain that the shell is no mere deposi- 

 tion of carbonate of lime. Like other cuticular products, 

 it has an organic basis (conchiolin or conchin), along with 

 which the lime is associated. 



Mr. Irvine's experiments at Gran ton 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. 



