No. 395.] REVERSAL OF CLEAVAGE IN ANCYLUS. 875 
before gastrulation occurs, and it seems impossible to connect 
the asymmetry of the adult with any of their divisions. The 
leotropic inclination of the arms of the cross in Planorbis is 
probably the expression of a torsion that affects, to a certain 
extent, a large part of the segmented ovum, and the reversed 
asymmetry of the adult may depend upon this general twist, 
rather than the reversed division of any of the cells of the 
entoderm. It is not improbable that the asymmetry of the 
animal may first manifest itself, as the results of various 
investigations indicate, in different parts of the embryo in 
different forms. 
The reversal of cleavage in Ancylus has, I believe, a special 
significance from the fact that the left-handedness occurring in 
this genus has, in all probability, arisen independently of that 
of Physa and. Planorbis. It may, indeed, be doubted whether 
the reversed asymmetry of the latter genera is due to their 
descent from a common sinistral ancestor; but, however this 
may be, it is scarcely possible that Physa, Planorbis, and the 
sinistral species of Ancylus all belong to one group which 
branched off from the dextral species of Ancylus and the other 
pulmonates. As is well known, we have sinistral individuals 
occurring as occasional variations among dextral forms, and 
in the genus Fulgur and in several genera of the terrestrial 
pulmonates there are well-established dextral and sinistral 
species. Analogous reversals of asymmetry occur in other 
groups of animals, such as the flounders, certain nematodes, and 
many crustaceans, and there appears to be no -great improba- 
bility, @ priori, in supposing an independent origin of left- 
handedness in Ancylus; besides, any other supposition would 
involve us in improbable phyletic derivations. The chance that 
the association of reversed cleavage with reversed asymmetry 
in the gasteropods is a mere. coincidence is very much 
lessened by the. circumstance that the same association 
obtains where the reversed asymmetry has been independ- 
ently acquired. It would be a matter of interest to ascertain 
if the cleavage of the dextral species of Ancylus were of the 
normal type, and if the right-handed and left-handed species 
of Fulgur and the terrestrial pulmonates, Pupa, Bulimnus, and 
