218 
BULLETIN OF THE BUREAU OF FISHERIES. 
During the early encystment stage this condition is constant. There is a uniform 
retention of the connective tissue involution, and as the mantle envelops it very 
closely in view of the pseudopodial nature of these cells this at once establishes a 
placental-like relationship. There seems to be, however, a certain shrinking action 
on the part of the involution, and in the late stages of development the foot fills 
up the mantle cavity so largely that the involution is necessarily pushed outward, 
as we have in Figures 7 and 8. 
SPECIALIZATION AS INDICATED BY MANTLE CELL CONDITIONS. 
In their relation to fish hosts it has been recognized that some mussels exhibit 
a lesser degree of dependence, which is related to the extent of specialization these 
species possess. With such species as Strophitus edentulus and Anodonta imbecillis 
this has been carried to the point where metamorphosis may take place in the 
absence of parasitism. These two forms would, therefore, represent the lowest 
degree of adaptation to parasitic life — that is, the least degree of degeneration — 
and this is indicated most clearly by the character of the mantle cells, so that a 
study of the mantle cell transformation of these two species will very likely reveal 
adaptations associated with a lack of parasitism. In the Anodontas there are 
forms that are said to occupy the middle ground of limited dependence upon fish; 
they must live on fish, but require little from them (Coker et al., 1921). With the 
mantle cells we observe a functional degeneration during parasitism that apparently 
is the result of lessened dependence, the mantle cells ceasing to function at an early 
period. This evidently is characteristic of fin attachments, since with these forms 
there is less diffusion of lymph into the cystic cavity, and consequently there is 
less need for any specialized structures for nutritional purposes. 
With the hookless glochidium, as typified by Lampsilis luteola, the closest rela- 
tion exists in its connection with the fish host. Being a gill parasite, this attach- 
ment affords it a more direct connection with the lymph stream, so that lymph 
diffusion takes place freely. Its very evident dependence on the fish host is reflected 
in the retention of the larval mantle as a nutritional organ, which is evidence that 
the nutritive process is in active operation and is a dominant factor in its larval 
development. This represents the more degenerate condition exhibited during 
metamorphosis, since the first evidence of specialization leading to greater depend- 
ence takes place with the larval mantle cells. The value of mantle cell studies is 
thus emphasized as a necessary preliminary to any study of larval relationships. 
LITERATURE CITED. 
Braun, M. 
1878. Ueber die postembryonale Entwicklung unserer Susswassermuscheln. Jahrbuch der 
deutscben malakozoologiscben Gesellscbaft, Jb. 5, pp. 307-319. Frankfurt a. M. 
1884. Ueber Entwicklung der Enten- oder Teichmuschel. Sitzungsbericbte der Dorpater Natur- 
forscher-Gesellschaft, Bd. 6, pp. 429-431. Dorpat. 
Churchill, E. P., jr. 
1915. The absorption of fat by fresh-water mussels. Biological Bulletin, Marine Biological Labora- 
tory, Woods Hole, Mass., Vol. XXIX, No. 1, pp. 68-86, 3 pis. Woods Hole. 
1916. The absorption of nutriment from solution by fresh-water mussels. Journal of Experimental 
Zoology, vol. 21, No. 3, pp. 403-430, 2 pis. Philadelphia. 
