426 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



for a decrease in its .size would lead to tlie exteruiinatioji ottlie 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 comjyarison of the branchial rhdm Iters. — The series of figures 

 representing cross-sections shows at a glance the c(ini]>arati\'e 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 Yolelia, Venus, Pecten, 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 divstance 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 fi'om an active ancestor, 

 with a fiilly 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 correspcmdingly 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, foi- 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 suflticiently buried in the sand the foot is «'ontracted and the greater part of 

 the blood is held by the mantle. The siphons, projecting into the water, are oj^en, and 

 a current is constantly running into the branchial chandler. 



Suppose the power of locomotion to have been lost aud the animal with its rudi- 

 mentary foot to have been fastened by a byssus, as in MytilKS. 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 witli the sexual gland, and the 

 branchial chamber is lessened in size. 



The <)yster 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 aud 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. 



