610 DR. R. V. EBRLANGER. 
disappeared, and the remaining left is homologous with the 
nephridial gland of other Monotocards. 
To the best of my knowledge, it is more probable that in 
most Monotocards the actual right kidney has disappeared, 
and that possibly it has been transformed into Perrier’s 
nephridial gland. 
A. Lang, in a short pamphlet purposing to explain the 
asymmetry of Gastropods by mechanical processes, has also 
arrived at the conclusion that in Monotocards the whole actual 
right complex of originally paired organs has disappeared. 
This pamphlet is evidently the result of Lang’s study of 
literature previous to the publication of the part of his text- 
book of comparative anatomy dealing with molluscs. Whilst 
most of the ideas expressed in Lang’s paper are certainly not 
new, his mechanical explanation of the asymmetry is certainly 
very ingenious. Biitschli, the last zoologist who dealt fully 
with the same question, had not attempted a mechanical 
explanation of this difficult problem. Unfortunately several 
facts are a serious impediment to Lang’stheory. For instance, 
it is difficult to understand why Trochus and Turbo, the 
shells of which are just as highly coiled as that of the common 
snail, should have retained two auricles and two kidneysif the 
pressure brought to bear by the shell on the actual right side 
of the body is held to have caused the disappearance of the 
corresponding set of organs. Our present knowledge of the 
development of Fissurella and Patella, which we owe to 
Boutan (2) and Patten,' is another serious objection, as it is 
well known that the shell of both these species is originally 
nautiloid. 
Another point which seems not to have met with any 
attention from Professor Lang is that up to the present it 
must have seemed highly probable that Perrier’s view on the 
homologies of the only remaining kidney in Monotocards, 
viz. that it represented the actual right one of Diotocards, 
must be correct. Lang neither mentions Perrier’s exhaustive 
1 Patten, ‘The Embryology of Patella,” in ‘ Arb. Zool. Inst. Wien,’ Bd. vi, 
1885, pp. 149—174, Taf. xiv—xviii. 
