426 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
for a decrease in its size would lead to the extermination of the species. As we have 
already seen, at the same time that the development of these fixing muscles was going 
on there was a corresponding atrophy of the foot, which was no longer needed. But 
the foot, in the exercise of its locomotor function, had been dependent upon the great 
blood spaces of the mantle. These gradually came to be of no value and disappeared, 
and the generative gland pushed out into the great folds from its crowded position 
in the visceral mass and completely filled them. 
Conclusions from a comparison of the branchial chambers. — The series of figures 
representing cross-sections shows at a glance the comparative size and conditions of 
the branchial chamber in the different forms. 
The very large mantle chambers seem to be characteristic of those forms which 
are most active, as Toldia , Venus, P 'eaten , and many others. In Mytilus, where the 
foot is much reduced in the adult, the surface of union of the mantle to the visceral 
mass is much greater, extending quite a distance down the side of the body, and so 
reducing the size of the branchial chamber. The foot has entirely disappeared in the 
oyster and the branchial chamber is much reduced, there being only room enough 
left for the short gills to lie between its folds (Figs. 2 and 4, PI. lxxix). 
This footless form is a very degenerate one, and came from an active ancestor, 
with a fully developed locomotor foot. The comparison of the branchial chamber 
is of interest in this connection. In an active locomotor form, like Venus and 
others, there would be a great deal of oxidation going on in the tissues, and this would 
necessitate correspondingly great facilities for the aeration of the blood. Conse- 
quently we find the gills greatly developed, and a large branchial chamber is neces- 
sary in which they may be suspended. Then, also, there being a large expanse of 
thin-walled mantle, whose interior is made up almost entirely of great blood spaces, 
there is a good chance for the aeration of the blood in them, for water from the exterior 
bathes the mantle lobes as well as the gills, at least on their inner surfaces. It may 
be objected to this that when the shell is open and the foot protruded, the blood 
is almost entirely absent from the mantle and present in the foot. But when the 
animal is sufficiently buried in the sand the foot is contracted and the greater part of 
the blood is held by the mantle. The siphons, projecting into the water, are open, and 
a current is constantly running into the branchial chamber. 
Suppose the power of locomotion to have been lost and the animal with its rudi- 
mentary foot to have been fastened by a byssus, as in Mytilus. Oxidation is lessened, 
for about the only tissues which sustain any wear are the large muscles which support 
the byssus from any shock which the animal may experience from the waves. The 
gills are proportionately smaller because less oxidation and less food are required ; the 
mantle lobes lose the blood spaces and become filled with the sexual gland, and the 
branchial chamber is lessened in size. 
The oyster has become fixed by a valve of its shell and has no byssus. The 
mantle blood spaces and the foot are both absent, and the form is capable of no 
movement whatever, excepting the closing of the shell and contraction of the mantle. 
The breaking down of tissues is thus reduced to a minimum and the need of the 
aeration of the blood is very slight indeed. So we find, as a result, an extremely small 
branchial chamber, admitting of small currents of water, for almost the entire space 
of the chamber is occupied by the gills. 
