THE COMMON MUSSELS. 11 



of the byssus are not quite so compact as in M. edulis (p rt, Figs. 

 30 and 31) — 



(1.) M. latus has no anterior adductor, while M. edulis has one. 



(2.) In M. latus the anterior retractors are retractors of the 

 byssus ; in M. edulis they are generally retractors of the foot. 



(3.) In M. latus the posterior retractors of the byssus are sepa- 

 rated into two bundles, the middle and the posterior retractors ; in 

 M. edidis the posterior retractors of the byssus are continuous. The 

 posterior retractors of the byssus are also much more compact in M. 

 latus than in M. edulis. 



Pallial Muscles. 



The margin of the mantle from the anterior end round to the 

 anterior part of the pericardium or the posterior end of the hinge is 

 furnished on the outer side with muscle-fibres set perpendicularly to 

 the margin. These fibres are called the pallial muscles, and in M. 

 latus this edging of muscle-fibre is very broad (Figs. 19 and 29), com- 

 pared with what exists in M. edidis (Fig. 31), and M. magellanicus 

 (Fig. 30). 



Alimentary Canal. (Figs. 13 to 16.) 



The alimentary canal of M. latus is of the type that is usual 

 among Lamellibranchs ; that is, it consists of a gullet or sesophagus 

 (Fig. 1) opening near the anterior end of the body, and leading into 

 a stomach provided with cseca. The intestine after leaving the 

 stomach makes several turns, finally opening in the anus over - the 

 posterior adductor (Fig. 1). The mouth, which is enclosed as in a 

 hood by the union of the mantle-lobes in front of it, is a transverse 

 slit surrounded by two pair of labial palps (Figs. 1 and 28). 



The gullet passes back obliquely upwards between the anterior 

 retractors of the byssus, a little above which lies the stomach list, 

 Fig. 1; stm, Figs. 13 and 14). The stomach is oval, and in great part 

 hidden by the hepatic cseca, whose ducts enter at all parts of the lower 

 half of the stomach. 



From the anterior part of the ventral side towards the left there 

 arises a small diverticulum or csecum coiled once on itself (Fig. 

 14a). This is the cardiac csecum. At the posterior end of the 

 stomach there are two openings, one above the other, from the upper 

 of which the end of the crystalline style often projects into the 

 stomach. The lower one is the opening of the intestine, the upper 

 that of the pyloric csecum which contains the crystalline style. 



The intestine (Fig. 1, and int, Figs. 13 and 14) passes back from 



