Affinity of Mollusea and MoUuscoida. By W. K. Brooks. 141 
or the mantle chamber in this class does away with the necessity 
for a locomotive embryo, but at present we know so little of the 
life history of the marine forms that we have very little ground for 
generalization. The imperfection of our present knowledge cannot, 
however, be fairly urged to restrain us from making as much use 
as possible of what knowledge we do possess, although we must 
constantly bear in mind that it introduces an element of uncertainty 
into all of our conclusions. This of course is true of all biological 
speculation at present, but no one would advocate the abandonment 
of all speculation and comparison until all of the facts of our science 
have been recorded and verij&ed. 
The embryo of Anodonta, at a very early stage, has, at the ante- 
rior end of the worm-like body, a simple band of cilia ; as develop- 
ment progresses this is carried, by the formation of the mantle lobes, 
into the mantle cavity, and there increases in length, and the free 
ends bend towards each other and finally unite, thus forming a 
closed, bilaterally lobed circlet like that of the Grasteropods, except 
that it is not raised from the surface of the body, and its cilia are 
very short and are not used for locomotion. It is interesting to 
notice also that it is attached to the dorsal surface of the shell by 
two muscles like those of the veliger of a Gasteropod. In Anodonta 
these subsequently become the retractor muscles of the foot. 
The thecosomatous Pteropoda present the veliger stage of 
development in a form as highly specialized as that of the marine 
Gasteropoda, and the embryos of the two do not differ at this time 
in any essential particular. The development of the gymnosoma- 
tous Pteropods, on the contrary, is entirely anomalous, and at 
present appears to be inexplicable on any theory of descent. 
In the Cephalopoda, as so often happens in the higher repre- 
sentatives of a group, the indirect course of development has given 
place to the direct ; the larval stages are usually entirely wanting, 
and the embryo shapes itself, from the beginning, into the form of 
the adult. In most Cephalopods there is no trace of a veliger 
stage, but its absence is what we should expect from the analogy of 
the higher forms of other groups. 
The conclusion to be drawn from our present knowledge of the 
Mollusea will appear, from this review, to be that all of them are 
to be traced back to a free-swimming ancestral form, of which the 
veliger embryo is the representative ; this seems to be the only way 
in which we can account for its appearance in at least certain repre- 
sentatives of so many widely separated groups ; and the presence of 
rudiments of it in such forms as Anodonta and the Pulmonates 
seems to indicate the same conclusion. We have seen that in many 
of the cases where it is wanting, its absence can be reconciled 
with this theory, even with our present knowledge, and we may 
therefore hope that a more complete acquaintance with the em- 
