XII 



PHYLUM MOLLUSCA 



649 



adductors are clearly marked on the inner surface (Fig. 538) 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) 



FIG. 538. Left valves of A, Mva: 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.) 



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. 548), 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 along the line of attachment of the gills, so as to 

 enclose a dorsal or exhalant siphon, a ventral or inhalant siphon 

 being formed simply by apposition 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 con- 

 vert the inhalant siphon into an actual circumscribed aperture or 



FIG. 539. Cardium edule. A, exhalant siphon ; B, inhalant siphon ; F, foot. (From the 



Cambridge Natural History). 



short tube. In the Sinupalliata the two siphons are prolonged 

 into distinct muscular tubes (Fig. 539, A. B) which, in the posi- 

 tion of extension, project beyond the posterior margin of the 

 shell, and may even be considerably longer than the body. Under 



