Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 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 att
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 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 Iiiiss 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 (conchyoliii), 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
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