THE LAMELLIBRANCHIA ote 
in adult Mytilidae, and disappears altogether in J/ytilus latus (Fig. 
193, E) and MW. meridionalis. 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 Lines adductor muscle is ventral and anterior to the 
anus (Figs. 188, h.a; 192, C, pa). 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- 
he 
Fig. 193. 
Right valve of six various Lamellibranchs, showing the various stages of the morphological 
development of the adductor muscles. A, Pectuneulus; B, Myrina; C, Modiolaria; D, 
Modiolu; E, Mytilus latus; F, Pecten. a, anterior adductor; a.r, anterior foot retractor; 0b, 
shell beak; li, ligament; p, posterior adductor; p.i, pallial impression ; p.r, p.7", 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 
