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 Thecosomatous Pteropoda are allied to Bulla, the Gymnosomatous forms to Aplysia.
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18 and 32), where the border of the mantle can be, and usually is, drawn over the shell, though it is withdrawn (as it cannot be in Aplysia) when they are irritated.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.
There is considerable uncertainty with respect to the names of the species of Aplysia.
The free edge of the mantle is seen just below the margin of the shell (compare with Aplysia, fig.
accessory glands and The generative organs lie close to the ducts of Aplysia.
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.
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.
48), contrasting greatly with what we have seen in Aplysia.
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.
- Nervous system of Aplysia, as a type of the longlooped Euthyneurous condition.
Visceral commissure much shortened, except in Aplysia.
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.
Cunningham, " On the Structure and Relations of the Kidney in Aplysia," Mitt.
(b) Ore antico; corpore pertuso laterali foraminulo: Limax, Aplysia, Doris, Tethis.
Accordingly, the shell of Aplysia must not be confounded with a primitive shell in its shell-sac, such as we find realized in the shells of Chiton and in the plugs which form in the remarkable transitory " shell-sac " or " shell-gland " of Molluscan embryos (see figs.
Aplysia, like other Mollusca, develops a primitive shellsac in its trochosphere stage of development, which disappears and is succeeded by a nautiloid shell (fig.
Thus the renal organ of Aplysia is shown to conform to the Molluscan type.
39) to the penis, and is by the aid of that eversible muscular organ introduced into the genital pore of a second Aplysia, whence it passes into the spermatheca, there to await the activity of the female element of the ovo-testis of this second Aplysia.
between a fp, fa, k, 1, P, of some days - possibly weeks - the ova of the second Aplysia commence to descend the hermaphrodite duct; they become en FIG.
The development of Aplysia from the egg presents many points of interest from the point of view of comparative embryology, but in relation to the morphology of the Opisthobranchia it is sufficient to point to the occurrence of a trochosphere and a veliger stage (fig.
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.
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