10 STUDIES IN BIOLOGY. 



says that in M. edulis "the anterior adductor is very small; it crosses 

 transversely the anterior hood of the mantle, and is inserted on the 

 inner face of each valve in an impression or groove at the anterior 

 extremity of the lower edge. The middle part of this muscle is 

 included in the thickening of the mantle." He speaks also of the 

 anterior adductor as " so small and rudimentary as to have been 

 overlooked by some observers." In the New Zealand specimens of 

 M. edulis it is so evident that only a very careless observer could fail 

 to notice it. [a ad, Figs. 5 and 11.) 



Retractors of M. edulis. Sabatier states that there are two re- 

 tractors of the foot, the anterior and the posterior. The anterior 

 retractors of the foot have exactly the position of those muscles that 

 I have above described as the anterior retractors of the byssus in M. 

 lotus, except that they are connected with the foot and not with the 

 byssus. But Sabatier adds that sometimes these anterior retractors 

 of the foot, instead of stopping at the foot, pass it and join the 

 muscles that work the byssus. What he describes as occasional in 

 M. edidis is the constant condition in M. laius ; that is, its anterior 

 retractors are not retractors of the foot, but of the byssus {a ft, 

 Figs. 9 and 10). The posterior retractors of the foot in M. edidis 

 are small flattened bands of muscle inserted below the pericardial 

 cavity. These muscles enter the foot and pertain exclusively to it. 

 These posterior retractors of the foot evidently correspond to the 

 muscles described above as the sole retractors of the foot in M. latus 

 (rft, Figs. 9 and 10), and the position of these last Justin front of the 

 middle retractors of the byssus shows that these middle retractors in 

 M. latus arise from the division of the posterior retractors, since in 

 M. edidis the posterior retractors of the foot are just in front of the 

 posterior retractors of the byssus. To show this more clearly, compare 

 Fig. 9 with Fig. 11, and suppose that the muscular bundle in the 

 latter (M. edidis) were divided and the parts separated : we should in 

 that case have a figure not unlike that in Fig. 9 (M. laius). 



" The retractors of the byssus in M. edulis are a series of mus- 

 cular bundles, setting out from the base of the byssus, which spread 

 out into a fan-shaped mass of four or five bundles, and which are 

 inserted on a lengthened, horizontal impression in front of the pos- 

 terior adductor and below the pericardial region." — Sabatiek. {p rt, 

 Fig. 81.) 



To summarise then the chief differences between the muscular 

 system of M. latus and that of M. edulis and mag ell aniens — for M. 

 magellanicus resembles M. edulis except that the posterior retractors 



