LAMELLIBRANCHIATA. 



327 



prismatic layer. It and the periostracum are secreted only by the 

 free edge of the mantle. 



The whole of the shell is a cuticular formation of the epi- 

 dermis, and its growth is effected in two ways: (1) by additions 

 to the nacreous layer whereby the shell increases in thickness; (2) by 

 additions to the prismatic and horny layers, whereby it increases 

 in superficial extent. Accordingly the outer coloured part of 

 the shell, which is com- 

 posed of vertical prisms 

 and the horny cuticle, 

 when once formed does 

 not increase in thickness ; 

 while new concentric layers 

 are continually being added 

 to the colourless nacreous 

 layer during the whole life 

 of the animal. It is this 

 nacreous secretion of the 

 mantle surface which, when 

 thrown down round foreign 

 objects which have worked 

 their way in between the 

 mantle and the shell, in 

 the so-called pearl oysters 

 (Meleagrina) and to a less 

 extent in other forms 

 (Unio, Margaritana), gives 

 rise to pearls. 



In some cases the valves do not meet ventrally, but always gape (Pholadidae, 

 'Jtixi'rochaenidae, etc. ). In exceptional cases the valves are fused dorsally (Pinna.). 

 The edges of the mantle are folded hack over the shell in the Galcommidaf 

 and in Entovaha. In some gaping forms the parts of the body projecting 

 beyond the shell secrete accessory pieces. Snch pieces may be independent 

 of the shell, as in the case of the dorsal pieces of Pholas (Fig. 270), and the 

 calcareous tube with which Teredo lines its burrow (Fig. 271), or fused to 

 the shell, as the calcareous tube of Aspergillum (Fig. 272). 



The skin consists of a slimy, one-layered epidermis, beneath 

 which lies a highly vascular connective tissue traversed by 

 abundant muscular fibres. The epidermis on the outer surface 

 of the mantle consists of columnar cells ; while on the inner sur- 

 face the cells composing it are ciliated (Fig. 253). Pigments are 

 present principally upon the edges- of the mantle, which are 



FIG. 253. Vertical section through the shell and 

 mantle of Anndonta (after Leydig). CM cuticle or 

 horny layer (periostracum) ; .S prismatic layer ; Bl 

 laminated nacreous layer (mother-of-pearl); Ef' 

 external epithelium of mantle ; Bd connective 

 tissue of mantle; Ep" internal epithelium of 

 mantle, ciliated. 



