1894-95.] Dr Gilchrist on Torsion of the Molluscan Body. 357 
On the Torsion of the Molluscan Body. By J. D. P. 
Gilchrist, M.A., B.Sc., Ph.D. Communicated hy Professor 
Ewart, F.R.S. 
(Read January 21, 1895.) 
There is strong evidence for supposing that the Mollusca are 
derived from a bilaterally symmetrical animal, having a median 
digestive tract and lateral excretory organs. This animal was 
further characterised by the physiological character of its dorsal 
integument, which had the property of secreting a peculiar sub- 
stance — conchiolin. This, though in itself capable of affording 
some protection to the animal, becomes impregnated with carbonate 
of lime, thus providing the animal with a hard coating sufficient 
to resist the attacks of all but the most powerful of its enemies, 
while at the same time it forms a strong external skeleton to which 
the muscles of the body can be attached. This conchiolin, how- 
ever, unlike chitin, does not seem to lend itself to the formation of 
jointed appendages or metameric segmentation, with which we are 
so familiar in another group, the Arthropoda. 
These are the primary facts on which the general features of the 
Mollusca are mapped out, and to which they owe their well circum- 
scribed character as a group. Other secondary factors come in 
which determine the classification of the sub-groups, and, as we 
shall see, tend to modify even the primary characters we have 
mentioned. These depend on the various modes in which the 
animal can utilise for the purpose of protection the calcareous 
plates or shells with which it is provided. One common mode is 
by a number of such plates along the dorsal surface, so arranged 
that even when the animal is detached from its support they are 
capable of being so folded together as to inclose the animal com- 
pletely — a mode of protection similar to that adopted by the 
armadillo among Mammalia. Mollusca protected in this way belong 
to the group which has been named the Polyplacophora. Such an 
arrangement evidently does not interfere with the primitive bilateral 
symmetry of the animal, and this group approaches nearer to the 
type of the primitive Mollusc than any other. 
