ANATOMY AND PHYSIOLOGY OF ATRINA RIGIDA. 
413 
The portion of the shell in the region of, and anterior to, the adductor is composed 
of two layers, there being a second or nacreous layer of the ordinary type deposited 
upon the inner surface of the prismatic. This layer is secreted by the general surface of 
that part of the mantle which lines the shell in these regions. The outer layer fre- 
quently wears through, or becomes brittle and broken, on the older portions of the 
shell, leaving the nacreous layer exposed. A discussion of experiments on the growth 
and regeneration of the shell will be found at the end of the next section. 
MANTLE. 
The mantle is a muscular membrane, the folds of which adhere closely to the shell, 
but are attached to it only at a single point just ventral to the adductor muscle. The 
muscles which control the ventral and posterior portions of the mantle are attached 
here and radiate from this point as divisions and subdivisions of a single bundle. 
Another bundle of muscle fibers is located near the dorsal part of the body. It is 
not attached to the shell at any point, but is inserted into the mantle itself. This 
bundle of mantle muscles also divides and subdivides into 
smaller and smaller bundles and is distributed to a portion 
of the posterior part of the mantle. (Fig. 16, pi. XLVIII.) 
It is thus seen that there is no pallial line in the shell for 
the attachment of the mantle muscles, though that is so 
common among lamellibranchs. Since the muscles are 
attached so high up, the mantle margin can be withdrawn 
a considerable distance from the edge of the shell; in fact, 
it can be withdrawn nearly to the adductor. After being 
contracted the mantle again expands by creeping outward 
upon the shell, to which it adheres closely. This result can 
not be brought about at once. At least half an hour is 
required for the mantle to again reach the edge of the shell after having been fully 
contracted. There are no siphons, but the two lobes of the mantle are united poste- 
riorly by an intermantle septum at the place where siphons might be expected to 
occur. This structure consists of two prominent ridges, one on each mantle lobe, which 
stretch across posterior to the gills to meet each other in the mid line. Each mantle 
ridge is continued anteriorly, though reduced in size, and forms the place of attach- 
ment for the upper borders of the reflexed lamellae of the outer gills. 
On account of the position assumed by Atrina, only the posterior portion of the 
mantle is exposed to frequent sensory stimulation. Connected with this fact we find 
that the edge of each mantle lobe has a row of short sensory tentacles, which decrease 
in size and gradually disappear toward the anterior end. This part of the mantle is 
thick and muscular, as an adaptation to burrowing. A deep narrow passage or groove, 
formed by the development of two tall ridges on the inner surface of the mantle, is also 
correlated with burrowing. (Fig. 16, D, pi. xlviii.) This groove lies parallel to the 
edge of the mantle and extends from the region of the foot to the intermantle septum, 
h 
Fig. 1. — The shell, a, Surface view; 
b, transverse section showing prismatic 
structure. 
