680 



ZOOLOGY 



SECT. 



great posterior adductor alone. In Anomia and in the Pseudo- 

 lamellibranchs there is a single immense adductor (Fig. 585, X, XI) 

 placed nearly in the middle of the greatly shortened body, and 

 known to represent the posterior adductor both from the fact that 

 the rectum passes over it, and from the circumstance that, in the 

 embryo Oyster, two adductors are present, the anterior of which 

 atrophies, while the posterior enlarges to form the single muscle 

 of the adult. 



These peculiarities in the muscular system bear their mark 

 upon the shell, in which impressions corresponding to the 



FIG. 586. Left valves of A, My a B, Modiola; C, Vulsella. The upper dotted line 

 passes through the hinge-lines, the lower connects the anterior and posterior adductor 

 muscles. (From the Cambridge Natural History.) 



adductors are clearly marked on the inner surface (Fig. 586). The 

 whole class is, in fact, frequently classified on this basis, species 

 with equal-sized adductors (Protobranchs, some Filibranchs, and 

 all Eulamellibranchs and Septibranchs) being called Isomyaria (A), 

 those with a large posterior and a reduced anterior adductor (most 

 Filibranchs) Heteromyaria (B), and those with large centrally 

 placed posterior and no anterior adductor (Pseudolamellibranchs 

 and Anomia among Filibranchs) Monomyaria (C). 



In many forms, such as Nucula (Fig. 596), Ostrea, &c., the right 

 and left mantle-lobes are quite free from each other, so that there 



are no siphons. 

 In Anodonta and 

 Unio, as we 

 have seen, the two 

 lobes unite so as 

 to enclose a 

 dorsal or exhalant 

 siphon, a ventral 

 or inhalant siphon 

 being formed 



Fid. 587. Cardium edule. A, exhalant siphon ; B, inhalant sml ply by appo- 

 siphon ; F, foot. (From the Cambridge Natural History.) sition of the lobes 



ventrally. In such cases the pallial muscles in their neighbourhood 

 act as retractors of the short and imperfect tubes thus formed. In 

 other species a second concrescence of the mantle-lobes takes place 

 so as to convert the inhalant siphon into an actual circumscribed 



