LAMKLLIBRANCHIATA. 



19 



the apex is usually shorter, and contains, at least in the equivalve 

 species, an excavation, the lunnla, by means of which the anterior 

 .-e'dge can be at once recognised. 



While the outer surface of the shell presents various sculpture 

 markings, the inner surface is smooth and shines with the lustre of 

 mother-of-pearl. On a closer examination, impressions and pits become 

 visible on the inner surface. A narrow line, the so-called mantle or 

 paUlal line (the line of attachment of the mantle edge to the shell), is 

 placed near and fairly parallel to the ventral edge of the shell (tig. 

 499, J//). In the siphoned forms this presents posteriorly a bend 

 directed forwards and upwards (J/7>). the pulUal bay, which is due to 



the siphons. Impressions are i ^_-^ = _ ===== 



usually caused by the inser- 

 tion of an anterior and pos- 

 terior adductor muscle which 

 I iiiss through the body of the 

 animal transversely from 

 one side to the other, and 

 are attached to the inner 

 surface of the shell (fig. 

 499, HM, FJ/). While in 

 the equivalve mussels (pr- 

 tJiaconclia} the two impres- 

 sions are usually of equal 

 size, in the uneq nival ve 

 forms (Pleuroconcha) the 

 anterior adductor is re- T 



FIG. 501. Vertical section through the shell and 



duced, and may Completely mantle of Anodonta (after Leydig). Cu, cuticle ; 

 i ,i_ - i S, prismatic layer; SI, laminated (mother-of-pearl) 



vanish ; the posterior ad- 

 ductor, on the other hand, 

 now a muscle of much larger 

 size, shifts forward to the middle of the shell (fig. 500). Hence 

 the names Dinti/urf// and Monomyaria. According to its chemical 

 composition, the shell consists of carbonate of lime and an organic 

 matrix (conchy oliii), which usually presents a laminated texture. 

 In addition to this laminated layer there is also a thick external 

 calt-areous layer, composed of large, pallisade-like prisms, which 

 are placed side by side and may be compared to the enamel of 

 teeth (fig. 501, S}. Finally, on the outer surface of the shell there 

 is a horny cuticle, the so-called epidermis (Cvi.}. The internal lami- 

 nated layer is secreted by the whole internal surface of the mantle, 



layer; Ep', external epithelium of mantle; Kd, 

 connective tissue substance : EJI", internal epi- 

 thelium of mantle. 



