412 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
surround the entire adductor muscle and visceral mass. In Ostrea, each of the four 
gills is connected above with the ones next to it, and the outer ones are also connected 
with the mantle. A vertical section (Fig. 2, PI. lxxix) shows each gill to be made of 
two lamellae ( ol and il), leaving a space (tv) between them. They are united to each 
other above, the top of the inner lamella of one to the top of the outer lamella of 
the next, and on the median line the opposed inner lamellae. In Fig. 2, which is a 
section near the anterior end of the gills, they are also all united above to the visceral 
mass. A little farther back, however (Fig. 4), the two inner lamellae of the inner gills 
only are attached to the body above. Farther back still, under the adductor (Fig. 8), 
the gills are entirely free from the parts above. In all regions, however, the outer 
lamellae of the outer gills are united with the mantle, as may be seen in all the figures 
of cross-sections of this form. 
By this concrescence of the gills above, the mantle chamber below is completely 
shut off from all the spaces which appear in section above them. Posteriorly these 
epibranchial chambers open into one another in one large cavity (Fig. 8, c), forming a 
cloacal chamber. Its position on the posterior side of the adductor muscle is shown 
at c (Fig. 97, PI. xciv). 
The gill lamellae are made of innumerable parallel filaments united to each other 
in various ways in different forms, and always leaving openings through which water 
may enter the epibranchial from the branchial chamber. This current is caused by 
the ciliated cells of the filaments. The spaces between the lamellae are called water 
tubes. The currents from the epibranchial chambers pass posteriorly to the cloaca. 
Into it, from above, opens the rectum in all cases (Fig. 97, r). In those forms which 
possess siphons, the cloaca opens into the anal siphon (Figs. 93 and 94). 
The more common position of the gills differs from that of the oyster, in that a 
foot is generally present, situated on the ventral side of the visceral mass and pro- 
truding between the inner gills. If this should occur in Ostrea and the gills should 
then be moved up on the sides of the body, we would have the condition in Venus 
represented in Fig. 14. The whole foot and visceral mass here separate the chambers 
of the right and left sides, instead of their being side by side, as in the former case. 
Behind the visceral mass and foot, the inner gills join one another, forming a branchial 
septum, which is continued posteriorly to the base of the siphons, and still preserves 
a complete separation between the cloaca and branchial chamber (Fig. 93, PI. xcm). 
Here also, as in Mya and many other forms, the epibranchial chamber is divided 
anteriorly into four parts, two on each side of the body, farther back into two, and 
finally these unite into one, the cloaca. 
In Mytilus there are no sharply defined epibranchial chambers, for, as may be seen 
in Figs. 32 to 41, the inner lamellae of the gills do not fasten to the body wall, nor do 
the outer lamellae unite with the mantle as they do in the forms just noticed. It thus 
happens that the water tubes (tv) open directly into the branchial chamber, whence 
their supply is obtained. The backward current is, however, confined to the dorsal 
part of the branchial chamber and leaves it by a special siphonal opening at its pos- 
terior extremity. (Fig. 87, PI. xcm, go, a view of the mantle edge in the posterior 
region.) 
Filamentous gills of this sort, often, however, undergoing great complications, are- 
possessed by the great majority of lamellibranchs. The gills of a few primitive forms 
(. Nucula , Yoldia , Solenomya), however, are entirely different in appearance. They were 
