[3] OYSTERS IN BAY OF ARCACHON. 9338 
mal then appears to have nearly the form of its shell, being usually 
oval in shape, and with its anterior extremity towards the narrow part 
of the shell. If after death, it is placed in water, so as to allow its 
parts to float, one sees in the center a mass of organs, separated by the 
contractor muscle noticed above, around which are attached large stri- 
ated and whitish lamin, which may be compared te the gills of fishes. 
Finally, the entire body is covered with a thin, transparent skin, whose 
edges are blackish in color and thickened, and correspond to the edges 
of the shell during the life of the animal. This membranous envelope, 
which is called the mantle, has its edges free and detached throughout 
almost the entire circumference of the body. On the sides it adheres 
to the principal organs constituting the abdominal mass. It serves at 
the same time to cover the oyster and to secrete its shell. Its thickened 
edges are of a muscular nature, and are also provided with secretory 
glands. Its main portion is formed of two very thin membranes 
united by a vascular tissue, which, when injected, presents to view a 
beautiful network of fine ramifications. In that portion of the mantle 
which is in contact with the shell there is also to be seen an organic 
web, in which calcareous granules are secreted in great abundance. It 
is these secretions, which, becoming detached, together with the organic 
matter enveloping them, increase the thickness of the shell, forming 
one by one the laminz above described. If one tries to open the man- 
tle he can turn back its lobes as far as the central muscle, but from 
here inwards to the anterior extremity, where the mouth is, it forms a 
sort of cowl inclosing the other organs of the mollusk. 
The mouth is recognized at once by its transverse position and by the 
two thin membranous lobes accompanying it. These lobes are con- 
tinued on each side of the body into a pair of narrow, lance-shaped 
labial palps or lips, which are smooth on the outside and marked with 
oblique striations on the inside. The mouth is a simple opening, which 
the animal can contract by means of a small sub-circular muscle; it is 
followed by a very short csophagus ending in an ovoid pouch with 
membranous walls (the stomach), into which open the bile ducts from 
the liver. A more important opening near the lower extremity of 
this stomach is the entrance to the pylorus. Here begins a slender 
intestine, which descends through the thick portion of the liver, in front 
of and somewhat under the muscle connecting the valves together; then 
it ascends obliquely towards the back, passes above the stomach till on 
a level with the mouth, and finally, bending upon its course, returns, 
passing over the upper side of the muscle, at the middle of which it ter- 
minates in an anus, situated between the lobes of the mantle. The liver, 
which is the principal organ of the oyster, constitutes a considerable 
portion of the visceral mass and is easily recognized by its dark green- 
ish color. The organs of circulation and respiration can only be exam- 
ined after they are injected with mercury. The heart is easily distin- 
guished by the blackish color of its auricle, and from the fact—very rare 
®” 
