Class 3. Acephala. 



311 



The shell increases in s i z e by the addition at its margin of new- 

 material, secreted by epithelial cells at the edge of the mantle ; it 

 increases also in thickness, since the outer surface of the mantle, 



Fig 257. Diagrammatic transverse sections through the shell of LameUibranchs with 

 internal {A, A') and external ligaments {B, B'). In A and B the shell is drawn 

 open, in A' and B' shut, b elastic portion of the ligament, 6' outer inelastic portion of the 

 same ; m adductor. — Orig. 



and, in the upper region, the surface of the body, continually deposit 

 new layers within it. 



Tte shells of some LameUitranclis differ from tlie usual type in being very 

 asymmetrical: in the Oyster and some Scallops, for instance, only one valve 

 is convex, tlie other being flat. A lesser asymmetry, quite insignificant in most 

 instances, occurs in many others, {e.g., the hinge-teeth of the two valves fit in 

 between one another ; at those places where a tooth is present in the one valve, 

 there must be a depression in the other). Other forms are characterised by the 

 fact that the shell covers only a small portion of the body {Teredo, and others). 

 Pearls ai-e calcareous deposits from the outer side of the mantle round foreign 

 bodies, which have got by accident between the mantle and the shell ; they are 

 either fii-mly attached to the inner side of the shell, or else lie quite freely ; they 

 occur in vai'ious forms. Some species, with elongate bodies and imperfect shells, 

 either build a tube of small foreign bodies welded together, or (more frequently) 

 secrete a calcareous tube, with which the small valves of the shell ai-e sometimes 

 united. 



In connection with the skin, the formation of byssus threads, 

 which occurs in one group, must be specially noticed. These threads 

 are horny fibres, which are secreted by the epidermal cells in a 

 depression of the foot, and in a groove connected with it. They 

 constitute organs of attachment in some species {e.g., in the Mussels), 

 one end remaining in connection with the animal, whilst the other 

 is firmly fixed to some foreign body. Others bind small stones 

 together by means of the byssus threads, and thus form a kind of 

 nest, in which they take up their abode. 



In many LameUibranchs, where no byssus is foimed, a nidimentaiy byssus 

 gland may nevertheless be present. Some forms possess this thread only in the 

 lai-vaJ stage, losing it when adult. 



