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.
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.
- Three views of Aplysia sp., in various conditions of expansion and retraction.
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.
18 and 32), where the border of the mantle can be, and usually is, drawn over the shell, though it is withdrawn (as it cannot be in Aplysia) when they are irritated.
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.
The free edge of the mantle is seen just below the margin of the shell (compare with Aplysia, fig.
In the nervous system of Aplysia the great ganglion-pairs are well developed and distinct.
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.
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.
For a comparison of various Opisthobranchs, Aplysia will be found to present a convenient starting-point.
44 and 45), are less abnormal than Aplysia in regard to their shells and the form of the visceral hump. They have naked spirally twisted shells which may be concealed from view in the living animal by the expansion and reflection of the parapodia, but are not enclosed by the mantle, whilst Actaeon is remarkable for possessing an operculum like that of so many Streptoneura.
The great development of the parapodia seen in Aplysia is usual in Tectibranchiate Opisthobranchs.
The chief modification of internal organization presented by these forms, as compared with Aplysia, is found in the condition of the alimentary canal.
Aplysia, Pleurobranchidium, Elysia, Polycera, Doris, Tergipes.
36) to the adult form has not been properly observed, and many interesting points as to the true nature of folds (whether parapodia or mantle or velum) have yet to be cleared up by a knowledge of such development in forms like Tethys, Doris, Phyllidia, &c. As in other Molluscan groups, we find even in closely-allied genera (for instance, in Aplysia and Pleurobranchidium, and other genera), the f operculum.
(b) Ore antico; corpore pertuso laterali foraminulo: Limax, Aplysia, Doris, Tethis.
Cunningham, " On the Structure and Relations of the Kidney in Aplysia," Mitt.
- Nervous system of Aplysia, as a type of the longlooped Euthyneurous condition.
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.