the common mussels. 9 



Muscular System. 



The muscles of M . latus for the more part may be divided into 

 two classes — those that serve to close the shell, the adductors, 

 and those that retract the foot or byssus, the retractors ; the latter 

 may again be subdivided, according as they move the foot or the 

 byssus, into retractors of the foot or retractors of the byssus. There 

 is considerable difference between M. latus and edulis with respect to 

 the arrangement of the retractors, and I shall describe first the muscles 

 of M. latus. 



Adductors. In M. latus there is but one adductor, the posterior 

 one, which is a large muscle, oval in transverse section. Its position 

 has already been described. There is no trace of an anterior adductor. 

 (Figs. 1, 9, 10, and 29.) 



Retractors . The retractors of the byssus form the great bulk of 

 the muscles, and of them there are three pair — the anterior, middle, 

 and posterior retractors of the byssus. The anterior retractors of 

 the byssus pass forward between the retractors of the foot, gradually 

 diverging from each other (a rt, Figs. 9 and 10), and ending nearly 

 opposite to the cerebral ganglia on either side of the supra-seso- 

 phageal cavity {a rt, Fig. 1) • they are slender rounded muscles. 

 The middle retractors of the byssus are two conical masses of 

 muscle-fibre attached to the shell by the wider end (m rt, Figs. 9 

 and 10), and standing almost immediately above the byssus. The 

 posterior retractors of the byssus are two stout muscles passing back 

 obliquely upwards to the upper anterior corners of the posterior 

 adductor (p rt, Figs. 9 and 10). The byssus is then the apex of 

 three muscular V s, its retractors ; the anterior V is nearly hori- 

 zontal, and is formed by the anterior retractors ; the middle V * s 

 nearly upright, and formed by the middle retractors ; the posterior V 

 leans on the upper corners of the posterior adductor, and is formed 

 by the posterior retractors of the byssus. The foot from its connec- 

 tion at its base with the byssus is of course affected by the contraction 

 of the byssal muscles; but the muscles directly connected with the 

 foot itself — that is, the retractors of the foot — are very small. The 

 retractors of the foot are two muscles not as thick as the anterior 

 retractors at their narrowest part, but widening out somewhat 

 towards their point of attachment to the shell. They curve up round 

 the anterior side of the middle retractors of the byssus, being closely 

 and flatly applied to these muscles, and attach themselves to the shell 

 just above them (r ft, Figs. 1, 9, and 10). 



In order to compare the muscles of M. latus and edulis I here 

 give Sabatier's (7) account of the muscular system of the latter. He 



