1894 - 95 .] Dr Gilchrist on Torsion of the Molluscan Body. 359 
Polyplacophora, the same phenomenon of the disappearance of 
the shell in forms which, from emhryological and anatomical 
evidence, we know to have been once possessed of a shell — as 
in the groups of the Cephalopods, the Pteropods, the Pulmonates, 
and the Opisthobranchs. The indirect reasons for dispensing 
with a shell have probably been the acquiring of other means of 
safety — such as new organs of defence and offence, protective re- 
semblance, &c. 
The question arises as to what becomes of the torsion of the body 
in those forms in which its mechanical cause (the shell) is removed. 
The chief object of the present paper is to note the results which 
have followed this process, which is none the less a true physio- 
logical experiment that it has been performed by nature. Is this 
twisting of the body undone ? Does the organism regain its primitive 
symmetry ? Do the atrophied organs reappear ? 
The group of the Opisthobranchs is peculiarly suited for this 
investigation, as we find in it all stages from the shelled to the 
completely shell-less forms. In describing the various changes in 
the topography which follow the removal of the shell, chief atten- 
tion will be paid to the position of the pallial cavity (shaded part 
in figures), the position of the gill (indicated by its line of 
attachment to the body), the position of the heart, and of the 
nephridial, genital, and anal apertures. A description of the neph- 
ridium itself would have been of great interest, but enough material 
was not at hand for this investigation. Due allowance must be 
made for possible contraction and contortion caused by fixing re- 
agents. The specimens examined were chiefly from Naples : some 
of the rarer forms were kindly sent me by Mr J. P. Hill, University 
of Sydney, and were useful for comparison. 
"We may note, in the first place, certain general features of the 
organism in the shelled forms. There are two parts of the body 
which must bear a definite relation to the shell, and therefore to 
each other, viz., the mantle which secretes the shell and the foot, 
which, along with the whole animal, can be withdrawn into the 
shell for protection. When, however, the shell disappears, these 
two organs are, as it were, set free to develop in any direction, and 
there is practically no limit to their possible transformation. As a 
matter of fact, we do find very different lines of development taken 
