But in Aplysia the mantle is reflected over the edge of the shell, and grows over its upper surface so as to completely enclose it, excepting at the small central area s where the naked shell is exposed.
For a comparison of various Opisthobranchs, Aplysia will be found to present a convenient starting-point.
From the fact that Aplysia commences its life as a free-swimming veliger with a nautiloid shell not enclosed in any way by the border of the mantle, it is clear that the enclosure of the shell in the adult is a secondary process.
In many respects the sea-hare (Aplysia), of which several species are known (some occurring on the English coast), serves as a convenient example of the fullest development of the organization characteristic of Opisthobranchia.
In the nervous system of Aplysia the great ganglion-pairs are well developed and distinct.
It is clear enough that the sac is of a different origin from that of Aplysia (described in the section treating of Opisthobranchia), being primitive instead of secondary.
Our figure of the nervous system of Aplysia does not give the small pair of buccal ganglia which are, as in all glossophorous Molluscs, present upon the nerves passing from the cerebral region to the odontophore.
The chief modification of internal organization presented by these forms, as compared with Aplysia, is found in the condition of the alimentary canal.
The detorted visceral commissure shows a tendency to the concentration of all its elements round the oesophagus, so that except in the Bullomorpha and in Aplysia the whole nervous system is aggregated in the cephalic region, either dorsally or ventrally.
The euthyneurous visceral loop is long, and presents only one ganglion (in Aplysia camelus, but two distinct ganglia joined to one another in Aplysia hybrida of the English coast), placed at its extreme limit, representing both the right and left visceral ganglia and the third or abdominal ganglion, which are so often separately present.
The great development of the parapodia seen in Aplysia is usual in Tectibranchiate Opisthobranchs.
The Thecosomatous Pteropoda are allied to Bulla, the Gymnosomatous forms to Aplysia.
There is considerable uncertainty with respect to the names of the species of Aplysia.
Thus the renal organ of Aplysia is shown to conform to the Molluscan type.
Visceral commissure much shortened, except in Aplysia.