MUSCULAR AND ALLIED SYSTEMS. 97 



C. 32. Right valve of a Mussel (Anodonta cygnea,), showing the 

 pallial, adductor, and pedal muscles. The former consists 

 of the muscular border of the mantle. The two adductors 

 of the valves are cylindrical muscles of equal size that pa?s 

 transversely from valve to valve at either end of the body. 

 The pedal muscles form the bulk of the foot and extend as 

 a thin envelope over the surface of the visceral mass ; 

 definite anterior and posterior pedal retractor muscles can 

 be distinguished from the general sheet, attached to the 

 valves directly behind the anterior and before the posterior 

 adductors. The contractile cortex of the individual muscle- 

 fibres is either longitudinally fibrillated or shows a double 

 diagonal stria ti on probably due to spiral fibrillation. 

 0. C. 52 A a. 



Engelmann, Arch. ges. Physiol., Bd. xxv. 1881, p. 553. 







C. 33. Right valve and muscular parts of an Edible Mussel 

 (Mytihis edulis). The anterior adductor of the valves is 

 considerally smaller than the posterior. The pedal muscles 

 are very strongly developed. They consist on either side 

 of an anterior retractor pedis, attached to the valve above 

 the anterior adductor, and a series of 6 or 7 posterior 

 retractors attached in a horizontal line anterior to the 

 posterior adductor. The posterior members of the series 

 are inserted upon the margins of the byssal gland. 0. C. 

 51 c. 



Sabatier, Ann. Sci. Nat., ser. 6, t. v. 1877, p. 9. 



C. 34. Right valve and muscles of a Clam (Mya arena mt). The 

 posterior part of the mantle is modified to form two mus- 

 cular tubes (the siphons) in which the fibres run in circular 

 radial and longitudinal directions. The longitudinal layer 

 is greatly developed, and forms a pair of definite siphonal 

 retractors. They have a very characteristic crescentic 

 attachment to the shell (pallial sinus), continuous with 

 that of the general mantle-border. Midway between the 

 anterior and posterior retractor pedis muscles is a small 

 elevator pedis. 0. C. 52 A b. 



C, 35. The valves of a Cockle ( Cardium edule) from which the 



