1894 - 95 .] Dr Gilchrist on Torsion of tlio Molluscan Body. 365 
Dolahella and Nofcarchus may be regarded as specialisations in 
the opposite direction where the pleuropodia, instead of climbing 
up the side of the animal as it were, increase so much both above 
and below the viscera as to envelop them like a sac. 
Lobiger is interesting, as showing a tendency of the pleuropodia 
to divide into lobes, perhaps foreshadowing the numerous dorsal pro- 
cesses of the Nudibranchs. 
There is nothing calling Tor special nbtice in the arrangement in 
Umbrella, which is adapted for a special mode of life — lying buried 
in mud and debris. The most striking features are ithe large foot 
and the development of the mantle edge into finger-like sensory and 
glandular processes, instead of being reflected over the shell. The 
great extension of the gill is also noteworthy, and the transforma- 
tion of the anterior part of it prepare us for the greater transforma- 
tion we meet with in the Nudibranchs. 
We have been following the line of development in which the 
anal, nephridial, and genital opening, along with the gill, tend to 
take up a median and posterior position, this being obviously in co- 
ordination with the growth of the pleuropodia (foot). A gap in the 
series occurs here ; and it is not till we pass over into the hfudi- 
branchs that we find further change in this direction, and these 
changes are then so radical as to throw doubts on the homology of 
some of the parts. For instance, in Polycera (fig. 6) the anal and 
Fig. 6. 
