250 MANUAL OF THE MOLLUSCA. 



wMcli is small ia Mijtiliis (fig. 30), but large iu Pbma, 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 ixmbones ' 

 (fig. 177, ««, ii,) or nearer the adductor, as in Astarte, and TJnio (fig. 172). ' 

 The posterior pair {p' p") are often close to the adductor, and leave no j 

 separate impression. The TJnionidce have two additional retractors of the ' 

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

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

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

 PI. XVII. fig. 21, 22.) j 



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

 moored by a strong byssus, 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 {b), and besides these two additional muscles {jhp) are developed. ! 

 In Puma, Modiola and Breissena the byssal muscles are equal to the great 1 

 adductors in size. J 



j 



Fig. 177. Muscles of Modiola.* \ 



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



The falciform processes of P/iolas and Teredo (PI. XXIII. fig. 19, 26) are \ 



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



i 

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

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

 muscles ; pp, bj'ssal muscles ; /, foot ; b, byssus ; ?«, pallial line. 



