310 ROBERT TRACY JACKSON ON THE 



moved from the hinge line than was the case with Modiola, and the antero-posterior axis 

 lies at an angle of abont 50" fi-om the hinge axis. 



A revolution of the axes similar to that traced in oysters, and in the above serial groups, 

 may be seen in Mulleria lobata, PI. xxvr, figs. 5-7, a widely separate genus. Accord- 

 ing to Adams and other authors, it has two muscles when young ; but, when fully grown^ 

 one muscle. The anterior muscle has disappeared, and the posterior, which is I'etained, 

 lies in the middle lower portion of the shell, in a position closely similar to that of an oys- 

 ter. This case is ])articularly interesting, as the change takes phice much later than in 

 the oyster; for the two-muscled stage is not an embryonic one, but is of a much later pe- 

 riod of development. In ^theria, a near ally, the two muscles are retained throughout 

 life, in a partially revolved position. Tridacna is another genus having but a single ad- 

 ductor muscle in the adult, and tliat muscle is the posterior. According to Woodward's 

 figure the single muscle is in a sub-centi"al position and the posterior I'etractor of the foot 

 occupies its common point of insertion close to the dorsal aspect of the adductor. The 

 mouth and palps have revolved dorsally so that they lie close up under the umbos as in 

 Ostrea, marking this as the anterior end of the valves, and a large byssus by which the 

 animal is attached is protruded through a notch in the valves immediately below the 

 umbos. The antero-jjosterior axis lies nearly perpendicularly through the umbos and 

 middle of the valves. The revolved position of the axes of the soft parts in Ti'idacna 

 is not accompanied by any marked changes in the foi-m of the shell excepting the posi- 

 tion of the byssal notch. Allies of Tridacna are typical dimyarians and it is doubtless 

 dimyarian when young. 



The theory of the revolution of the axes explains the changed position of the axes 

 and adductor muscles on purely mechanical grounds. It is found in widely separate 

 genera of Pelecypoda that the revolving of the axes is accompanied by a reduction and 

 final loss of the anterior adductor as it moves into a position close to the hinge area 

 where its mechanical action is slightly if at all effectual in closing the valves. At 

 the same time the single posterior adductor increases in size as it moves into a sub-cen- 

 tral position where its action is most effectual. The revolution of the axes in Pelecy2)ods, 

 relativelj' to the axis (hinge) of motion of tlie valves, is therefore seen to be closely cor- 

 related with the rethiction and final loss of the anterior adductor, together with the in- 

 crease and final exclusive retention of the posterior. This theory substantially as here 

 presented was published in my preliminary paper. While that paper was in press, a paper 

 was published by Dr. Benjamin Sliarp in which he presents a mechanical theory for the 

 loss of the anterior and retention of the posterior adductor muscle in Pelecypoda, which 

 is quite similar to that of the revolution of the axes. 



It is a very noticeable fact that oyster spats attached to various foreign bodies do 

 not lie with their axes arranged in any special direction. Ditterent individuals attached 

 to the same stone or shell, may have their axes at every angle of deviation, in the plane 

 of the surface of attachment, as compared with one another. I have never seen them ar- 

 ranged with any reference to the resistance of prevailing cui-rents. On the other hand, 

 Anomias and Crepidulasare sensitive to prevailing currents. Large numbers of Anomias, 

 attached to a single block, will very connnonly be found with their axes parallel and 

 headed one way (see discussion on development of Anomia, section xiii), and Crepi- 



