314 TRANSACTIONS OF THE [aPR. 12, 



shell, to change the position of the same, now here and now 

 there, so as to make a tube of sufficient roominess. It is further- 

 more able to change the direction of the burrow with great 

 readiness, the sliape of the shell offering no obstacle. 



The mechanisms of the molluscs which form the subject of 

 this paper atlord an interesting and important contribution to 

 the discussion in regard to the origin of the adductor muscles 

 in the pelecypod molluscs as set forth in the chapter on Kineto- 

 genesis in Professor E. D. Cope's latest work " Primary Factors 

 of Organic Evolution." Professor Cope here quotes Dr. R. T. 

 Jackson at length. Dr. Jackson points out the facts in the or- 

 dinary dimyarians, of which Mya may be taken as a type, and 

 in the derivative monomyarians (^Ostrsea, Peclen). According 

 to this authority, if the axis passing through the two adductor 

 muscles (the antero-posterior axis) is parallel to the longitudinal 

 axis of the hinge ligament, which coincides with the axis of mo- 

 tion in these forms, the adductor muscles are about equal in size. 

 As the antero-posterior axis rotates the anterior .ndductor dimin- 

 ishes in size, and the posterior adductor, which is now between 

 and in a line with the hinge ligament and the posterior adductor, 

 comes into a position of poor mechanical advantage and " disap- 

 pears from disuse and atrophy." This is the condition in which 

 the monomyarian oyster and pecten ai'e found. 



In the piddocks another condition of affairs obtains. The 

 antero-posterior axis has rotated so as to intersect the hinge axis, 

 but here the point of contact of the valves has passed between 

 the anterior and posterior adductors, which are still synergetic 

 but with the effect of opening the shell in front and below in- 

 stead of closing it. The opponents of these muscles are the 

 ventral adductor and the anterior portion of the anterior adduc- 

 tor, respectively, and these are likewise synergetic with the 

 effect of closing the valves in front and below. It must be 

 understood that the contractions of these muscles are probably 

 not exactly synchronous, but serial. This does not, however, 

 alter the mechanical conditions. In other words, the axis of 

 motion, which maintains a constant position in such forms as 

 Mya and Peclen, swings in the manner already described. 



By reference to the figures it will be seen that the four ad- 

 ductor muscles form a quadrangle within which is found the 

 point of contact. The diagonal passing through the anterior ad- 

 ductor and the ventral adductor passes through the point of 

 contact and constitutes an axis of motion for the adductor 

 muscles on either side of it. The other diagonal would play the 

 same part for the anterior and ventral adductors. It is evident 

 that these two Jines mark the limits between which the actual 

 axis of motion swings. 



