ANATOMY AND PHYSIOLOGY OF ATRINA RIGIDA. 
417 
its function. So long as a specimen is undisturbed this glandular structure is likely to 
lie quietly in the cloacal chamber, but when the mantle is irritated, for example by break- 
ing off part of the shell, it becomes active and moves about in every conceivable direction. 
It was frequently pushed far down into the branchial chamber toward the point of irri- 
tation. When grains of sand were put upon the mantle this muscular gland sometimes 
succeeded in brushing them off after several trials and much aimless maneuvering. While 
this organ is moving about the glandular tip is usually kept pressed against the mantle and 
appears to be sweeping its surface. It seems to be a “ swab ” for the purpose of freeing the 
mantle of any foreign body which may lodge upon it. 
Just why the Pinnidse need such a structure isdifficult to determine, since other lamel- 
libranchs get on without it, the cilia on the inner surface of the mantle being equal to 
the task of keeping it clean. The position assumed by Atrina is one of disadvantage for 
removing debris. It has been pointed out that great quantities of dirt and sand do 
enter the mantle chamber, and this must all be raised vertically to the edge of the shell 
for expulsion, so that structures especially adapted to this purpose are to be expected. 
The mantle gland is probably such a structure. 
The mantle gland is much less compact in structure than the foot. On the outside 
there is an epithelial covering which is glandular only at the tip of the organ. Here 
the cells are very much elongated and they contain a large amount of secretion in the 
form of granules. Immediately beneath the epithelium there is a band, or cylinder, of 
longitudinal muscle fibers. They are attached to the organs at the base of the gland 
for support. Many of them spread out over the adductor, into which they are inserted. 
They are so distributed in the gland that they can control the direction of its movement 
provided that they do not all contract at the same time. The shortening of the gland 
is also brought about by the contraction of these muscles. The central part of the gland 
is composed of very open connective tissue and a few transverse muscle fibers. 
LABIAL PALPS. 
The palps consist of two thin muscular lamellae which extend across the anterior end 
of the body, one above and the other below the mouth. Their outer ends are roughly 
triangular in shape and lie alongside the body. The epithelium lining the palps is 
continuous with that of the mouth and Drew (2) has aptly likened these structures to a 
pair of drawn-out lips. They are essentially alike in many lamellibranchs but vary 
greatly in size and shape in different species. In Atrina they are comparatively large 
and consist of two well-defined portions. That part which lies near the mouth is narrow 
and is lined by a smooth ciliated epithelium, while the outer triangular portion is broad 
and is lined by an epithelium that is thrown into a series of prominent ridges and grooves 
large enough to be plainly seen without magnification. Posteriorly the palps inclose the 
anterior ends of the gills, and it is their function to transport the food collected by these 
organs to the mouth. An extra projecting membrane is present on the ventral border 
of the inner palp, which folds up over the outer. (Fig. 16, pi. xlviii.) 
48299°— Bull. 29 — 11 27 
