Affinity of MoUusca and Molluscoida. By W. K. Broohs. 137 
stages. In the Gasteropoda these tentacles remain free from each 
other during the whole life, and the water circulates over and 
around them ; while in the Lamellibranchs they become so bent 
upon themselves and united to each other, that the gill-tubes are 
formed, and the water is driven into and through these, to be dis- 
charged into the cloaca, which is a special chamber, peculiar to the 
Lamellibranchs. In such a form as Mytilus, where the union 
between the tentacles is somewhat imperfect, we have what appears 
to be an intermediate stage between the perfect lamella of Mya or 
Unio and the separate tentacles of a Gasteropod. The gills of a 
Lamellibranch are therefore, like the shell and the digestive organs, 
a specialized form of the embryonic type, which is pretty closely 
adhered to in the adult Gasteropod. 
These facts must not be regarded as showing that the Lamelli- 
branchs are higher than or derived directly from the Gasteropods, 
for any such conclusion is rendered impossible by the lack in the 
latter group of such peculiarities as the lingual ribbon ; a cen- 
tralized and highly evolved nervous system ; and accessory organs 
of reproduction. Although it is true that these features might 
have been lost through adaptation to a sedentary hfe, their entire 
absence at all stages of growth, throughout the whole class, would 
seem to indicate that they never existed ; so we cannot derive these 
animals directly from the Gasteropoda, but must regard them as an 
offshoot from a form of which the Gasteropods are the highly deve- 
loped linear or nearly linear descendants. If this conclusion is 
accepted, it is plain that all attempts to trace the phylogeny of the 
higher Mollusca through the Lamellibranchs to the Molluscoida, 
must be erroneous and useless. 
The history of the discussion of the affinities of the Mollusca 
is an almost unbroken record of generalizations based upon imper- 
fect knowledge and erroneous conceptions, and so many arrange- 
ments of the group have been proposed, accepted for a time, and 
then shown to be unnatural, that it is not at all strange that many 
naturalists should now call in question the existence of any real 
affinity between the higher and the lower classes. As long as the 
attention of the investigator was confined to the study of shells, 
there seemed to be no difficulty in connecting the Lamellibranchs 
with the Brachiopods through such forms as Anemia ; and although 
the slightest anatomical knowledge is sufficient to show that the 
resemblance between these forms is entirely superficial and without 
scientific value, this conception had been so generally accepted and 
so firmly established, that the confirmation by embryology of the 
results reached through anatomical research, has scarcely been able 
to thoroughly exterminate it. 
This view has been replaced by another which is not open to the 
charge of superficiality, since it is based upon a thorough know- 
l2 
