250 MANUAL OF THE MOLLUSCA. 



which is small in Mytilus (fig. 30), but large in Pinna. The retractor 

 muscles of the foot (already alluded to at p. 26) have their fixed points near 

 those of the adductors ; the anterior pair are attached within the umbones 

 (fig. 177, , ,) or nearer the adductor, as in Astarte, and JJnio (fig. 172). 

 The posterior pair (p 1 p') are often close to the adductor, and leave no 

 separate impression. The Unionidce have two additional retractors of the 

 foot, attached laterally behind the anterior adductors ; in Leda, Solenella y 

 and a few others, this lateral attachment forms a line extending from the 

 anterior adductor backwards into the umbonal region of the shell. (See 

 PI. XVII. fig. 21, 22.) 



In those shellfish like Pinna and the mussel, which are permanently 

 moored by a strong bt/ssus, the foot (/) serves only to mould and fix the 

 threads of which it is formed. The fibres of the foot-muscles pass chiefly to 

 the byssus (d), and besides these two additional muscles (p,p) are developed. 

 In Pinna, Modiola and Dreissena the byssal muscles are equal to the great 

 adductors in size. 



Fig. 177. Muscles of Modiola.* 



In a few rare instances the muscles are fixed to prominent apophyses. 

 The falciform processes of Pholas and Teredo (PL XXIII. fig. 19, 26) are 

 developed for the attachment of the foot-muscle; the posterior muscular 



* Fig, 177. Muscular system of Modiola modiolus, L. from a drawing communi- 

 cated by A. Hancock, Esq. aa, anterior, a* a 1 posterior adductors; uu and p'p', pedal 

 muscles; pp, byssal muscles ; /, foot; b, byssus; m, pallial line. 



