3. On some Points in the Anatomy of the Nervous System of the Pond Snails, Planorbis and Lymnæus

Extract

The best figures of the nervous system of these animals are those of Lacaze-Duthiers, in his Paper Sur les Otocystes des Mollusques, and in that Sur le Système Nerveux des Gastéropodes. By a careful study of the nervous system of these two gasteropods, chiefly by cutting sections, I have discovered one or two small points in their anatomy which, as far as I can make out, have hitherto escaped attention.

These two types, though so closely allied, differ to a considerable extent in the arrangement and structure of certain of their ganglia; compare the figures given by Lacaze-Duthiers of the nervous system of Lymnæus on Plate xvii. in Volume i. of his Archives with that cf Planorbis on Plate iii. of the same volume. It will be seen at once that the cerebral ganglion of Lymnæus is far more complicated than that of Planorbis, consisting of five separate lobes, while that of Planorbis appears to be made up of not more than two.