358 Proceedings of Royal Society of Edinhurgli. [sess. 
It may happen, however, that the animal takes to other modes 
of life, or acquires other means of defence, and the old and some- 
what inconvenient mode of protection is rendered superfluous, and 
is dispensed with. Thus we find a group — the Ajplacophora — 
where in the adult all traces of a shell are lost. 
Another well marked division of the Mollusca is constituted by 
those forms protected by two shells only. These are laterally 
placed, and by being closely approximated can afford a secure protec- 
tion for the animal which lies in the space between them. Such 
an arrangement would be, of course, impracticable in a crawling 
animal, but is well adapted to the peculiar mode of feeding of this 
animal. Such a mode of protection has a modifying effect on the 
head and foot region, but the bilateral symmetry of the animal is 
still retained. This group might be called the Di;placopliora. Some 
of these forms (e.y., Galeomma) may be regarded as exhibiting a ten- 
dency towards an aplacophorous condition. 
By far the greater majority of the Mollusca, however, are animals 
which possess, or have possessed, a single shell as a means of protec- 
tion. This group might be called the Monoflacofhora. The shell, 
being capable at all times of containing the animal, must necessarily 
be in the form of a gradually enlarging tube. In its growth it may 
increase equally all round its edge, in which case it assumes a conical 
shape j or it may grow more at one side than the other, in which 
case it has a coiled form of a definite geometrical shape. In animals 
which have a burrowing mode of life (Dentalium), or are free- 
swimming (Xautilus), the carrying of this heavy burden has little 
or no effect on the bilateral symmetry of the body ; but in the case 
of the Gasteropoda or crawling forms, where the animal has to bear 
the weight of its shell, an entirely new factor is introduced, viz., 
a torsion of the body. At the side, usually the left, on which the 
shell bears most heavily, the gill, nephridium, &c., either become 
wholly atrophied, or change their position so as to lie on the 
opposite side. There is a corresponding change in the organs of the 
right side, which pass over towards the left ; and in the anus, which 
now comes to occupy an anterior position. Spengel has pointed 
out a striking confirmation of this in the crossing of the pleuro- 
visceral nerves brought about by this torsion. 
In this group — the Monoplacophora — we find, as we did in the 
