31 



fibrils beiiif^ thrown into folds by contrnotioii, produoinor 

 an apparent striation only. Transverse striation has also 

 been observed in Pecten operciddris, on the ctenidial 

 muscles (figf. 45, Br. m.), the appearance here beinp^ 

 exactly as in the mantle. Both cases are probably due to 

 contraction. 



The Circular Muscles run parallel to the margin of 

 the mantle and are very well developed in the Yelum 

 (fig. 4. V . M . c), which is made up almost wholly of these 

 muscle fibres. When Pecten closes its valves rapidly, 

 whilst swimmino:, the water between the valves must 

 endeavour to escape at the ventral margin by forcing the 

 two vela apart. One can see, then, the use of this develop- 

 ment of circular muscles, because if the vela are kept in 

 a rigid condition by their action, the water will be com- 

 pelled to pass out at each side dorsally. near tlie hinge 

 line, as previously described. These circular muscles are 

 inserted into the shell in conspicuous bundles anteriorly 

 and posteriorly (fig. 3, V. M . a.) at the same level as the 

 fusion of the mantle lobes. 



The Retractor Muscle of the foot is the posterior 

 retractor of the left side, and is the sole representative of 

 the four retractor muscles which attach the foot and con- 

 tained viscera to the shell in the majority of lamelli- 

 branchs. In monomyarian forms, the two anterior 

 retractors are usually absent, but Pecten has gone further, 

 and, moreover, the single retractor which is obvious in 

 P. opercnlaris has become even more vestigial in P. 

 maximus. 



In both species the attachment to the shell is in the 

 same position, along the dorsal margin of the adductor 

 muscle, near the junction of its two parts, and the 

 retractor impression ou the shell cannot be distinguished 

 fion) that of the adductor. 



