FORMATION OP SHELL OF MOLLUSCA. 331 



and is frequently composed, where it does exist, of nothing but 

 a thin horny plate, destitute of calcareous deposit. 



942. As the mouth of the shell is always at its widest part, 

 and as, in extending it, the cavity is enlarged as well as its 

 entrance, there is evidently no necessity for such a division into 

 separate plates, with a provision for the individual growth of 

 each, as we have seen in the Cirrhopoda ( 897) ; nor for the 

 periodical exuviation and renewal which is performed by the 

 Crustacea. In order to adapt its size to the progressive increase 

 in the bulk of the tody, new layers are deposited from the mantle 

 at intervals, each of which usually lines the previous one, and 

 extends beyond it. The portion which thus projects is generally 

 thicker and firmer than the rest of the new layer ; since that part 

 which is deposited within the previous layers is protected and 

 supported by them. And the corresponding part of the mantle 

 is also thicker and more spongy, so as to possess almost a glan- 

 dular texture. At this part of it, indeed, are situated the glands, 

 by which the colouring matter is secreted, that gives to the 

 exterior of the shell its beautiful and variegated tints ; these are 

 wanting in that portion of the mantle, which merely forms the 

 lining to the older layers. In general we find each new layer 

 in immediate apposition with the last ; but this is not always 

 the case. The animal forms it upon the mould, as it were, of 

 its own body ; and if this has shrunk, or changed its form, so as 

 not to occupy the whole cavity of the shell, a space will inter- 

 vene. This is remarkably shown in the Spondylus varius, or 

 Water- Clam, a section of whose shell exhibits a regular series 

 of chambers thus formed ; and these are usually filled with 

 water. In the common Oyster such spaces may often be ob- 

 served ; but they possess no regularity. The animal always 

 appears inclined to adapt its shell to the form of the body, by 

 reducing its cavity if necessary, as well as by extending it ; and 

 thus an Oyster, which has been kept without food, and whose 

 body has thereby shrunk, so as not to fill the interior of the 

 shell, will expend its last energies in forming a new layer 

 adapting the interior surface to its altered condition. The 

 texture of the shell varies considerably in the different tribes of 



