66 
tional importance, since they may be omitted occasionally in 
the oyster, and normally in Unio and Anodonta. In Massa 
we find them again, but they are here associated with the 
presence of a food-yolk, and I think we are, therefore, justi- 
fied in concluding that the one-sided, bilaterally symmetrical 
segmentation which there is occasion to regard as character- 
istic of the Lamellibranchs, indicates that the Lamellibranchs 
are the descendants of an ancestral form, in which the eggs 
were few, large and provided with a food-yolk ; that this has 
been lost, as the eggs became small and numerous, but that 
the peculiar form of segmentation which was then necessary 
has been retained perfectly by the oyster, and incompletely by 
other Lamellibranchs. 
In a paper which was printed several years ago, (The Af- 
finity of the Mollusca and Molluscoida, Proc, Boston Soc. 
Hat. Hist. XVIII, Feb. 2, 1876, pp. 225-235), I called atten- 
tion to a number of reasons for holding the opinion that the 
Lamellibranchs must be regarded as a side branch from the 
main stem, of which the Gfasteropods are a much more direct 
continuation, and that all attempts to trace the phytogeny of 
the higher Mollusca through the Lamellibranchs to lower 
invertebrates are erroneous and useless ; that the highly spe- 
cialized “veliger” of the marine Prosobranchs is to be re- 
garded as the proto-mollusc, and that the Gasteropods are de- 
scended from this with less modification than the Lamelli- 
branchs. The growth of our knowledge of the invertebrates 
has furnished us with much more material for comparative 
study than was available at the time this paper was written, 
and seems to indicate very clearly that the ciliated embryos 
of the Echinoderms, Gephyreans, Annelids, Polyzoa, Brachi- 
opods, Botifera, Molluscs and other invertebrates are, all of 
them, modifications of a common ancestral type, and that the 
origin of these great groups is indicated by their embryology. 
At the same time the careful comparison of adult animals 
has directed attention to the fact that, in many cases, those 
groups in which the structure of a type is reduced to its sim- 
plest expression, are not ancestral, but degraded, forms. In 
