98 PHYSIOLOGICAL SERIES. 



ventral parts have been removed, to show the fibres of 

 the two adductor muscles. 0. C. 52. Hunterian. 



C. 36. A vertical section through the umbo of a Cockle (Cardium 

 edule] . The viscera have been removed to show the posterior 

 adductor and posterior retractor pedis muscles. 0. C. 52 A. 



C. 37. Two specimens of Pholas [Dactyllna] dactylus, giving a 

 superficial and median view of the muscular system. The 

 anterior adductor muscle lies almost entirely external to the 

 hinge-line, attached to a special upward reduplication of 

 the shell, and in consequence its contraction produces an 

 abduction of the valves, the elastic ligament that generally 

 performs this function being absent. An additional ad- 

 ductor of the valves is formed by an enlargement of the 

 postero-ventral border of the mantle, In the lower 

 specimen a fan-shaped anterior retractor pedis muscle is 

 shown attached to the anterior and ventral margins of 

 the falciform process. 0. C. 53 D. 



Egger, Arb. Inst. Wiirzburg, Bd. viii. 1888, p. 151. 



C. 38. Right valve of a Scallop (Pecten maximus) with the chief 

 muscles attached. One adductor (the posterior) alone is 

 present, it is readily divisible into two portions a large 

 anterior part o a greyish colour, and a very much smaller, 

 glistening white posterior part. The grey portion is com- 

 posed of transversely striated muscle-fibres. The white 

 portion consists of longitudinally fibrillated smooth fibres. 

 Experiments show that the grey portion is capable of very 

 rapid and transitory, the white of slow and prolonged 

 contraction. 



The striped muscle is the main agent in swimming, by 

 the sudden closure of the valves a number of times in rapid 

 succession. It also probably serves, as in the case of 

 Terebratella, for the swift closure of the valves in case of 

 danger, the almost tendinous smooth muscle maintaining 

 the closure. 



The pedal muscles are represented by a much reduced 

 left posterior retractor. 0. C. 51 B. 



Blanchard, Bull. Soc. Zool. France, t. xiii. p. 76 ; Knoll, 

 S.B. Akad. Wien, Bd. ci. Abt. 3, 1892, p. 481. 



