Only a few papers and works can be mentioned here, with the remark that few authors have paid attention to the all-important innervation of the muscles.
Here we need only further draw attention to the osphradium, discovered by Lacaze-Duthiers, and shown by Spengel to agree in its innervation with that organ in all other Gastropoda.
This very effective and elaborate innervation, which has been directly traced to the brain, whence strong nerves (generally two) enter the proboscis, renders it exceedingly probable that the most important functions of the proboscis are of a sensiferous, tactile nature.
It stretches forward as far as the brain, and in Carinella is again continued in front of it, whereas in the Heteronemertines the innervation of the anterior extremity of the head, in front of the brain, takes the form of more definite and less numerous branching stems. The presence of this plexus in connexion with the central stems, sending out nervous filaments amongst the muscles, explains the absence, in Pro-, Mesoand Heteronemertines, of separate and distinct peripheral nerve stems springing from the central stems innervating the different organs and body-regions, the only exceptions being the L.N.
Optici, for the innervation of the eyes.
He emphasized the importance of our active experiences of movement and effort, and though his theory of a central innervation sense is no longer held as he propounded it, its value as a suggestion to later psychologists is great.
The otocysts are invaginations of the epithelium of the foot, but are innervated from the cerebral ganglia, and the same innervation has been proved in some cases for the osphradia.
As regards their innervation an apparent exception is found in the case of A pus, where the nerves to the antennules arise, behind the brain, from the oesophageal commissures, but this is, no doubt, a secondary condition, and the nerve-fibres have been traced forwards to centres within the brain.