THE LAMELLIBRANCHIA 



211 



in adult Mytilidae, and disappears altogether in Mytilus latm (Fig. 

 193, E) and M. mtridionalis. It is very small or more usually absent 

 in the Anomiidae, the Pectinacea, and the Ostraeacea, much reduced 

 in Teredo, and absent in the adults of Philobrya, Miilleria, and 

 Tridacna. 



The posterior adductor muscle is ventral and anterior to the 

 anus (Figs. 188, h.a ; 192, C, p.a). When the anterior adductor is 

 diminished in size or disappears in the adult, the posterior adductor 

 necessarily becomes more central in order that its mechanical 

 efficiency may be increased. This may be seen in the forms known 

 as Monomyaria, and is accompanied by a shortening of the antero- 



Ilight valve of six various Lamellibranchs, showing the various stages of the morphological 

 development of the adductor muscles. A, Pectunmlun ; B, Myrina ; C, Modiolaria ; D, 

 Modiola ; E, Mytilus latus ; F, Pecten. a, anterior adductor ; a.r, anterior foot retractor ; b, 

 shell beak; li, ligament; />, posterior adductor; p.i, pallial impression; p.r, p.r', posterior 

 retractors of foot and byssus ; t, hinge-tooth. 



posterior axis and a proportional increase of the dorso- ventral axis 

 of the body, a phenomenon particularly well marked in the 

 Tridacnidae. It should be observed that the species with a single 

 adductor muscle belong to very various groups and are generally 

 sessile forms : the Monomyaria, therefore, are polyphyletic and 

 do not constitute a natural group. A single family may contain 

 examples of Isomyaria, Anisomyaria, and Monomyaria (Fig. 193, 

 C, D, E), and all, in the course of their development, pass through 

 three different stages with regard to the arrangement of the adductor 

 muscles. In the first stage, called the protomonomyarian stage, the 

 anterior adductor, being the first to be formed, is alone present. 

 In the second stage the two adductors coexist ; this is the dimyarian 



