364 Proceedings of Boyal Society of Edinhurgli. [sess. 
Taking first the forms in which the foot preponderates, we come 
to a number of molluscs of which Aplysia is a type. The forms 
examined were various species of Aplysia, Dolabrifera, Dolahella, 
Notarchus, and Lobiger. These do not differ in any important 
detail as to the position of the pallial cavity, and fig. 5 (of Aplysia) 
will serve to give a general idea of the position of the pallial organs 
of the whole. These are all situated on the dorsal part of the 
animal, in contrast to the previous group, where they had a tendency 
to be turned towards the right side of, and even partly under, the 
visceral mass. There is here no dragging backwards of any part, as 
indicated specially by the position of the heart. In Aplysia the 
pallial organs are crowded into the middle dorsal position by the 
surrounding pleuropodia, thus approaching a well-known Nudb 
branch arrangement (fig. 6). This is even more marked in Dola- 
brifera, where the pleuropodia (more particularly the left) tend to 
disappear, and the “ pallial complex ” is much reduced, as in the 
Nudibranchs, the resemblance to which is still further increased by 
the posterior position of the mantle cavity. Several species of 
Aplysia examined showed, on the other hand, transition stages 
between the position met with in the Cephalaspidea and that de- 
scribed here for Aplysia limacina, A. depilans, &c. This was found 
to be the case in A. piperata, A. brasiliana, &c. 
