398 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Iii the oyster the partition between the two openings in the mantle is permanent, 
the mantle folds being united. Fig. 89, PI. xciii, represents a view of this part of the 
mantle seen from behind. The mantle folds are fused at brp. Above, the upper part 
of the cloacal chamber is exposed where it communicates with the exterior, and the 
rectum (r) is seen to open into it. Below the fusion, the branchial chamber is seen, 
together with the posterior ends of the gills ( og and ig), which hang in it. 
In Mytilus this region is more modified. The cloacal opening, which is small, 
appears from the exterior to be much more definite (Fig. 87, PI. xcih, eo) and is sur- 
mounted by a tough ring, which, like the whole mantle edge in this region, is colored 
by a deep-brown pigment. Tentacles are here absent from the mantle edge. Ventral 
to this, the mantle folds do not shut in a complete branchial orifice. Closing the upper 
part of the space between the mantle folds on the inside, is a thick, pigmented mem- 
brane, a part of the mantle (br m). This is also shown in Fig. 88, which is a pos- 
terior view of Fig. 87. In this figure the extreme posterior ends of the gills mark the 
partition between the epibranchial chamber above, whose opening to the exterior is 
seen at co f and the branchial chamber below. Thus far there is little indication of 
a development of much tissue in connection with the siphon-like openings other than 
that usually present in the mantle edge. 
If we imagine that the two separated openings in Ostrea and Mytilus are made 
definite — that is, that the branchial passage is separated from the ventral mantle edge 
by a second fusion — and that they are protruded as tubes, we will have in the main 
the condition in the siphoned forms (Fig. 96, PI. xciv, sn). A vertical, longitudinal 
section through the posterior region of the body of Venus (Fig. 93, PI. xciii) will show 
the relations of most of these parts. Below, and to the right in the figure, is seen the 
branchial chamber (br c), in which hang the gills (ig). The upper part of the gills, 
forming the floor of the epibranchial chamber (ep c), is seen in the section, and the 
openings of the water tubes into the latter chamber are indicated. Bounding this 
part of the epibranchial chamber above is the posterior adductor (pa). The epibran- 
chial chamber opens into the base of the cloacal siphon, which also receives the end of 
the rectum (r) from above, it having come down over the. adductor. This basal por- 
tion of the siphon may perhaps be called the cloaca, though it is small. The lower 
or branchial siphon opens into the branchial chamber, but at its base there stretches 
across, its whole upper part a membrane (br to), occupying the same position as that 
already referred to in Mytilus. If we should take an unsectioned specimen and throw 
back the mouth folds so as to get a view of the base of the lower siphon from the 
branchial chamber, we would have the membrane shown as in Fig. 90, PL xciii, br to. 
Extending down from the posterior end of the gills, it covers all but the lower part of 
the base of the branchial siphon seen below in the figure. It does not extend straight 
across from one side to the other, but presents the appearance of a deep notch extend- 
ing upward. On either side of it the bases of the thick siphon walls may be seen. 
One might naturally suppose that from its position this branchial membrane 
would allow water to enter the branchial chamber, but that any back flow would 
apply it over the base of the branchial siphon; this would prevent the water from 
escaping, thus acting as a valve. If the animal is able or has occasion to use this 
fold in such a way, I have not observed it. Mactra solidissima, the sea clam, much 
like Venus in anatomical points, has a branchial membrane that apparently completely 
Covers the branchial siphon at its base. But if an individual be quickly taken out of 
