392 BUT.LETIN OF THE UNITED STATES FISH COMMISSION. 



face, though this is verv doubtful. These striated fibers {smf) ramify, from their 

 attachment to this membraae, in toward the center of the auricle. They are generally 

 in irregular ])uudles, though not in the sense that the fibers are closely bound to one 

 another. The fibers are very long. 



Scattered among them are many single pigment cells {pfjc), Avhich give the auricle 

 its brown color. Also surrounding the fibers are many cells, apparently of an epi- 

 thelial nature, which seem to ])e giving off vacuolated ends (v) into irregularly formed 

 spaces, thus having very much the same appearance as the secreting cells of the 

 nephridium. Their nature needs further study. 



The functions performed by these two portions of the adductor mentioned above 

 are not well understood. Von Jheriug (Xo. 23) thinks that the part with the plain 

 fibers is used simply as a means of keeping the valves from spreading too far apart 

 on account of the action of the shell ligament. The same view is held by Lankester 

 (No. S), who does not say, however, which portion he believes to exercise this function. 

 In opposing the view of Von Jliering, Pelseneer (Xo. 17) says that in the FJwladidce, 

 which have no ligament between the valves of the shell, the adductors are formed 

 entirely of fibers which have no appearance of striation. He thinks that it is proba- 

 ble that, when these two i)aits exist, "la partie a apparence striee des adducteurs 

 sert, comme chez les autres invertebres, a produire des contractions rapides." 



Although 1 had made very few observations on the subject, I had come to a dif- 

 ferent conclusion from any of these. It seems to me that the fibers of the darker por- 

 tion of the adductor muscles are more compact and firm, and probably supply the 

 greater part of the force required in keeping the shell closed. The fibers of the lighter 

 portion, not being packed so closely together, are able to contract more quickly and 

 close the shell, it may be against a sudden attack. If a valve of Pecten be removed, 

 the smaller darker area will be seen to preserve an extremely contracted condition, 

 while the large white part, also partly contracted, now and then makes very sudden 

 and violent contractions, and then immediately relaxes somewhat. These contractions 

 can be made to occur by striking or cutting almost any part of the body besides the 

 adductor, but more especially the mantle edge. The contractions also occur from time 

 to time if the animal is undisturbed. It seems then that a sudden closing of the shell, 

 so often necessary to lamellibranchs, is accomplished by the lighter portion, and that 

 the darker part comes more actively into play when the shell is to be closed for some 

 little time. The lighter portion in Pecten is relatively very greatly developed, and, as 

 the very large size of the adductor has been brought about partly for locomotion by its 

 extremely rapid contraction, the lighter part is the one which performs this function. 

 Though other forms do not have this method of locomotion, the manner of contraction 

 of the white and dark fibers may be the same. 



The two pairs of foot-retractor muscles are of general occurrence, except in forms 

 with an aborted or absent foot, and are well seen in Venus. They are attached to the 

 shell close above the adductors (Fig. 96, PI. xciv, afr and ^)/r), and join the anterior 

 and posterior parts of the foot, respectively. The anterior ones, w^hich pass obliquely 

 backward from their attachment, are shown cut across at ar, Fig. 11, PI. lxxxi, which 

 is a vertical section in the region of the anterior cud of the stomach. In Fig. 12, a little 

 farther back, they are cut more nearly longitudinally and show their final union with 

 one another and the foot, /. The same relations are shown for the posterior retractors 

 in Venus in Fig. 17, PI. Lxxxii, atjjr. 



