vii MOLLUSCA MUSCULATURE 125 



transversely striated, although their striation does not correspond with that of 

 Arthropod and Vertebrate muscles. 



The pedal musculature, taken as a whole, answers to the 

 columellar muscle of other Molluscs, especially of the Gastropoda. 

 It consists of symmetrical pairs of muscles attached at one end to the 

 inner surface of the shell on which they leave impressions, the other 

 ends entering the foot. The correspondence of this musculature with 

 the columellar muscle of the Gastropoda is best seen by comparing a 

 Protobranchiate with Patella or Fissurella. In Nucula or Leda, for 

 example, there is an almost continuous series of muscle bundles 

 running down to the foot on each side between the anterior and 

 posterior adductors. The two series taken together, seen from above 

 or below, have an oval outline answering to the horseshoe-shaped or 

 almost oval form of the section of the columellar muscle in Patella 

 (Fig. 106) or Fissurella. 



In most cases in which the foot is developed, the following muscles on each side 

 are distinguished in order from before backwards (cf. Fig. 108) : (1) the protractor 



AA 



-OB 



FIG. 108. Pliodon Spekei, from the left (after Pelseneer). The shell, mantle, gills, and oral 

 lobes of the left sides removed. AA, Anterior ; AP, posterior adductor ; OA, anal ; OB, branchial 

 aperture of the siphon ; V, visceral mass ; p, foot ; 1, protractor pedis ; 2, retractor pedis anterior; 

 3, elevator pedis ; 4, retractor pedis posterior. 



pedis ; (2) the anterior retractor pedis ; (3) the elevator pedis, and (4) the posterior 

 retractor pedis. 



Where there is a byssus, the posterior retractor becomes the byssus muscle. It 

 is then usually highly developed, runs far forward, and may break up into several 

 bundles. 



In those cases in which the foot is rudimentary and the byssus wanting, the pedal 

 muscles degenerate. 



In Pecten the pedal retractors are asymmetrically attached, i.e. only to the 

 left valve. The same is the case in Anomia, where the shelly plug which lies in 

 the byssus notch of the right valve, and corresponds with the byssus, is attached to 

 the left (or physiologically upper) valve by two highly-developed retractors. These 

 two muscles leave scars near that of the adductors. This fact gave rise to the 

 erroneous opinion that the Anomia were Trimyaria. 



