MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 391 



Internally the valves are white or colored in various tints. In some cases the 

 coloration is due to pigment in the shell substance, in others to the refraction of light 

 by the striated prismatic surface. In the former case, when the color is bright, it may 

 fade at the death of the animal, as I have seen it do in certain species of Unio. 



The positions of the attachment of the adductor muscles to the shell are indicated 

 by glossy, more or less oval areas, which are sometimes, as in Ostrea and Mytilus, 

 darkly colored. Very often {Venus, My a, Mactra, etc.) a line of the same glistening 

 appearance runs along near and parallel to the ventral border of the shell, joining at 

 either extremity the adductor muscle scars. This pallial line generally folds inward 

 posteriorly in those forms possessing siphons, making a deep loop into which they 

 may be retracted. The mantle edge is attached to the shell, though not very firmly, 

 throughout the extent of the pallial line. 



The shell has sometimes been made the chief or only basis for the classification 

 of the members of the Lamellibranchiata. But a single organ is a very unsafe basis 

 for the comparisons needed in such a case. 



ADDUCTOE AND RETRACTOR MUSCLES. 



The adductor muscles connect one valve of the shell with the other and are gen- 

 erally very conspicuous. In order to open the shell these must be cut away from their 

 connection with it. Each adductor is generally made of two kinds of fibers, one being 

 of a darker shade than the other. The darker portion of the muscle is generally inte- 

 rior to the lighter, and is in some cases larger, in others smaller, than the latter. The 

 former condition is shown in Venus and Ostrea (Figs. 96 and 97, PI. xciv) and the latter 

 in Fig. 47, PI. Lxxxvi, which rej)resents the adductor of Pecten irradians, cut longi- 

 tudinally and vertically. The darker portion, very much contracted, is shown above 

 the more extensive lighter part. It is well known that, especially in the forms of the 

 3fonomya, this darker portion of the adductor is made up of fibers presenting a stri- 

 ated appearance, which, however, does not correspond to the cross striation in muscle 

 fibers of arthropods and vertebrates. The lighter i)art of the adductor is made up 

 of i)lain, unstriped fibers. Pelseneer (No. 17) confirms the supposition that this condi- 

 tion also exists in many forms with two adductors, Avhich he has examined. 



This striation of muscle fibers, Avhich has been referred to by many writers as 

 being present in the adductor fibers of lamellibranchs, has not been described in 

 definite terms, but is generally spoken of as an '' appearance of striation." So far as 

 I know, it has only been seen in the adductor muscles. I have found in the muscles 

 of the heart of Ostrea virginiana a remarkably distinct striation of the fibers. I have 

 not yet had the time at my disposal for a careful histological examination of the nature 

 of this striation, though it promises to be a ftworable subject for such study, as the 

 striations are large and not numerous, there being about twenty to .03 millimeters 

 of the fiber. The muscles of the auricles of this heart at least seem to be composed 

 entirely of these striated fibers. 



Fig. 65, a cross-section of a portion of the auricle of the heart in Ostrea, will give 

 some idea of the nature of this striation. Sm represents an apparently homogeneous 

 basement membrane of the many-layered epithelial wall of the auricle. At certain 

 places it makes a loop out beyond the wall {a), perhaps gaining some support to 

 resist, in this way, the contractions of the muscle fibers which are attached to its inner 



