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.
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.
For a comparison of various Opisthobranchs, Aplysia will be found to present a convenient starting-point.
In the nervous system of Aplysia the great ganglion-pairs are well developed and distinct.
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 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.
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 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.
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.
Thus the renal organ of Aplysia is shown to conform to the Molluscan type.