MUSCULAR SYSTEM. 



The muscles of each of the organs are best described in con- 

 nection with the structure of the organ concerned. As the 

 muscle that functions in closing the shell is the only one that 

 would not naturally receive attention in describing the organs, 

 it will be necessary here to describe in detail only the adductor 

 muscle. 



There is but one adductor muscle in the adult scallop and this 

 corresponds to the posterior muscle of those forms that possess 

 two muscles. In the very young scallop the anteriqr adductor 

 muscle is present (fig. 35, aam.) and for a time is the only 

 functional one, but the posterior muscle soon makes its appear- 

 ance. At what stage of development the anterior adductor 

 muscle is lost has not been determined but a scallop a centimetre 

 (half an inch) in diameter shows no indication of such a muscle. 



In the adult scallop the adductor muscle has a greater area 

 of attachment on the left, upper, valve than on the right, and 

 the scar on the shell is comparatively smooth and indistinctly 

 bounded. Usually there are some markings indicating the 

 division of the muscle into bundles but they are not nearly as 

 prominent as on the other valve. 



The muscle is quite definitely divided into two portions. An 

 anterior large rounded portion (fig. 10, pa.) and a posterior 

 somewhat crescent shaped portion (pa'.) that is applied on its 

 concave side to the anterior portion. The left end of the pos- 

 terior portion is a somewhat narrower and longer crescent than 

 the right end and is applied much more closely to the anterior 

 portion of the muscle than at the right end (figs. 9 and 

 16). Where the muscle is attached to the right shell 

 valve the separation of the anterior and posterior portions is 

 marked by a deep cleft on the ventral side of the muscle. This 

 cleft extends along the ventral side of the muscle for nearly half 

 the length of the muscle where it becomes a very distinct line 

 that may readily be followed to the other end of the muscle. 

 The two portions of the muscle differ in color, the posterior 

 portion being darker, and their physiology is quite different. 

 The large anterior portion may be entirely severed and the pos- 

 terior will close the shell with nearly as great rapidity as was the 

 case before the muscle was injured, but if the posterior portion 



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