* 
67 
such forms as the Lernseans and Entoconcha the degradation 
is due to actual parasitism, but degradation may be effected 
"by any circumstances which diminish the complexity of the 
environment and thus render a simplification of structure ad- 
vantageous ; and the view that the characteristics which are 
most distinctive of the Lamellibranchs have been produced 
as adaptations to their sedentary life, and that their remote 
ancestors were similar to those of the higher molluscs, is sup- 
ported by ample analogy. 
It is clear that as an animal becomes adapted to a sedentary 
life its diffusion must be provided for by an increase in the 
number of eggs or embryos, and we can easily see that, if the 
ancestors of the Lamellibranchs were animals which laid only 
a few large eggs, the gradual acquisition of a sedentary habit 
would demand a decrease in fhe size of the eggs, in order to 
permit an increase in their number. 
The evidence which the oyster egg furnishes to show that it 
was at one time provided with a food-yolk, which has been 
lost, therefore confirms tne view that the Lamellibranchs are 
a degraded or simplified group. 
I take the opportunity, while correcting the proofs of this 
paper, to call attention to a highly interesting discussion of a 
kindred subject, which is contained in an elaborate paper, by 
Rabl, on the development of Planorbis, in the last number of 
the Morphologisches Jahrbuch, which reached me after this 
paper was written. 
Rabl gives a comparative table of figures, in part original 
and in part selected, of the early stages in the segmentation 
of the eggs of a number of Gasteropods, Heteropods and 
Pteropods, and shows that there is a complete series of forms 
between the radially symmetrical segmentation of the Plan- 
orbis egg, and the egg of Hassa. 
His series of figures fully proves his conclusion that the pe- 
on liar segmentation of Hassa has been brought about by the 
gradual localization of a specialized food-yolk, and I think all 
embryologists will agree with him in holding that his facts 
show the fundamental similarity in plan of segmentation 
among the Gasteropods and Pteropods. 
