Affinity of MoUusea and MoUuseoida. By W. K. Brooks. 135 
expressed diluted juice of horse-dung, it conclusively proves to me 
that the resting spores hibernate naturally in the same manner. 
The seat of danger from both parasites is clearly in dungheaps, 
ditch sides, and decaying Potato plants. 
Any method of destroying the resting spores of these pests, or 
of warding off or mitigating their attacks, obviously depends in a 
great measure upon a full knowledge of their life history. That 
life history I have endeavoured to the best of my ability to watch 
and describe, and I am content to let the observations stand on 
their own merits. Sensibly conducted and extensive field experi- 
ments might probably teach some valuable lessons; but it is 
difiicult, if not impossible, for any single individual, whether 
farmer or botanist, to institute and carry out such experiments. — 
Gardeners' Chronicle, pp. 39-42, 1876. 
IV. — The Affinity of the Mollusca and MoUuscoida. 
By W. K. Brooks, Ph.D. 
During last August and September (1875) I enjoyed, through the 
kindness of M. Agassiz, an opportunity of studying the development 
of several of our more common marine Gasteropoda ; and the results 
reached seem to point to the conclusion, which I believe has never 
been pointed out, that although the Gasteropoda are much more 
specialized and highly evolved than the LamelHbranchs, nearly all 
their organs, excepting those of locomotion and relation, conform 
much more closely to the embryonic type than do the same organs 
in an adult Lamellibranch. The latter group must therefore be 
regarded as a side branch from the main stem, of which the Gaste- 
ropoda are a much more direct continuation. 
1 have already shown * that the embryonic shell of Anodonta is, 
at first, a cup covering what is to become the dorsal surface of the 
embryo, and is therefore homologous with the shell of a Gasteropod. 
This cup or hood soon folds down on to the sides of the embryo, 
precisely as described in Dentalium by Lacaze-Duthiers, and at a 
very early period splits along the dorsal median line and becomes 
separated into the two halves of a bivalve shell, which are thus 
shown to be together the homologue of the shell of a Gasteropod 
exclusive of the operculum, which, as Selenka has shown in his 
' Entwickelung von Tergipes claviger,' is formed by a split which 
extends across the long axis of the body, and therefore at right 
angles to that which, in Anodonta, gives rise to the two valves. 
The valves of an adult lamellibranchiate shell are a specialization 
* ' Proc. Amer. Association,' 1875. 
VOL. XVI, L 
