Fig. 368.—Longitudinal section of Pleurobranchus aurantiacus, showing circulation and gills or branchiæ.—(Warne.)
In the Opisthobranchs the branched veins as well as the auricle are placed behind the ventricle of the heart. They differ from Nudibranchs inasmuch as they are usually furnished with a pair of tentacles and labial palpi, or an expansion of the skin like the veil of the larval form. To clearly understand the character of the internal organisation of these curious animals, the longitudinal section given in [Fig. 368] must be consulted: p is the foot; a the mouth, covered above with the veil-like expansion, over which are the tentacles, c; the branchial veins, v, carry the blood to the gills, from which it flows into the heart at h. This disposition is the opposite of that which characterises the Prosobranchus. Another anatomical peculiarity, which may here be referred to, is the direct communication of the system of blood vessels with the surrounding medium; a characteristic common to most other molluscs, and on which depends the changeable external appearance of the animal. In the illustration of Pleurobranchus here given, g indicates the opening of the duct which conveys water direct to the blood, and through which the blood vessels permeate the back and foot. Like the holes in the sponges, it can be filled or emptied at the will of the animal.
Although this, in the main, is the principle of the circulation in most of this order, one branch possesses no special breathing organs, respiration being carried on throughout the naked skin of the body.
With regard to the Nudibranchiata, the group having the most symmetrical form is the extensive family Dorididæ, characterised by differences in the branchiæ, the relative proportion of the mantle to the foot, and variations in the radula and jaws. The general aspect of the genus Doris, although drawn on a small scale, is represented in [Plate XVII]., Fig. b. The whole sub-order of Nudibranchs has become more generally known and admired since the publication of Alder and Hancock’s monograph with its many attractive coloured illustrations.
These gastropods can be kept alive for some time in a small aquarium if the precaution is observed of often changing the water and adding a little fresh seaweed. Numerous curious microscopic forms of life may be found adhering to them.
Fig. 369.—Aplysia dipilans.
Tunicata.—The most remarkable group of animals belonging to this sub-order are the Ascidians. They derive their name from the test or tunic, a membranous consistence, in which they dwell, and which often includes calcareous spicules. The test has two orifices, within which is the mantle. Few microscopic spectacles are more interesting than the circulation along this network of muslin-like fabric, and that of the ciliary movement by which the fluid is kept moving. In the transparent species, as Clavelina and Perophora, the ciliary movement is seen to greater advantage. The animals are found adhering to the broad fronds of fuci near low water-mark. They thrive in tanks, and multiply both by fission and budding. Two species are figured in [Plate XVII]., Figs. i and k, the zooids of which were found arranged in clusters, as represented.
Aplysiidæ (sea-hares), so called on account of a slight resemblance to a crouching hare. The body form is elongated with a partially developed neck and head, oral and dorsal tentacles, and furnished beneath the mantle with a shelly plate to protect the branchiæ. The mouth is provided with horny jaws, and the gizzard is armed with spines, to prepare the food for digestion. The side lobes are thin and large, and are either folded over the back or used in swimming. [Fig. 369] is a reduced drawing of A. dipilans.