In the cirrigrada, locomotion is effected the ciliated arches to form eight ganglia, from each of which two nerves go to the adjoining bands, and one, larger than the others, runs upwards in the middle of the transparent space between the bands, and can be traced to be yond the middle of the body. In the course of these last-mentioned nerves, two or three smaller ganglia are visible, from which fila ments pass inwards to the viscera. Dr. Grant likens these nerves and filaments to the abdo minal nerves of pectinaria and other transpa rent animals.
The circular fibres forming the sphincters of the orifices of the air-bladder in physalus have been mistaken for nerves.* There is no evidence that the acalepha possess any other sense than that of touch. But, al though they cannot be said to have the sense of sight, they are evidently affected by light. At least some of the smaller tribes shun a bright light, and sink into the deep to escape from it.
In most of the tribes of acalepliw, the sense of touch seems to have its seat chiefly in the tentacula and cirri, with which almost all are provided. The degree of sensitiveness with which these are endowed varies much. In some, the slightest touch, even agitation of the water, is sufficient to excite them to contrac tion. These organs of touch, as has been al ready mentioned, are subservient chiefly to the nutritive functions. Other parts of the bodies of most acalephw also manifest, by their con tractions, a certain degree of sensitiveness. Several of the ciliograda alter the shape of their general mass when touched. In physalus the crest appears to be more sensitive than any other part. Many species, particularly of the pulmograda, give no signs of their feeling even the deepest and most extensive wounds of their discs. But it was observed by Spallanzani, that, by friction, and by punctures of the mus cular membrane of the disc, the movements of contraction and dilatation could be excited in medus?, which, having been kept in a dry place during twenty-four hours, had discon tinued their ordinary motions, and had lost nearly two-thirds of their bulk by the running out of their contained fluids.-1 III. Digestion.—The structure and action of the organs concerned in the function of digestion in the acalepha are still involved in much obscurity. Even in the large and fre quently examined physalus, it is difficult to ascertain the functions of the various parts in a satisfactory manner; and, accordingly, there exists so much difference of opinion amongst anatomists with regard to them, that some will not even admit that it has a mouth, while others assign to it both a mouth and an anus, as well as ccecal prolongations of the stomach.
Eschscholtz concluded, from his numerous ob servations on the living animals, that, in all the physograda, the digestive organs consist merely of absorbing tubes or suckers, all of which are simple, and pendent from the inferior sur face. He seemed to think that the action of these filamentary organs was analogous to that of the roots of plants ;—that they were en dowed with an endosmosic power, which en abled them to imbibe nutritious matter from the water. However this may be with regard to the simple filaments, or cirri, it appears pro bable that the suckers are provided with orifices at their extremities, through which proper ali mentary matter passes into the interior; for several observers agree in stating, that both the physograda, and the diphyda apply their suckers to the bodies of other animals, and re main adherent to them for some time, during which they seem to take up some nourishing matter. Eudoxia has only one sucker. Messrs. Quoy and Gaimard have described in detail the singular filamentary organ which bears these suckers in diphyes. Generally it is seen, at first, only as a shapeless opaque mass, of a reddish colour, lying contracted within the swimming cavity. But, gradually, it is ex tended, and then there are perceptible, along the whole of one side of a fine transparent tube, numerous suckers, of a lengthened form ; each is covered by a very delicate bell-shaped case, and has its base surrounded by groups of mi nute vesicles, which are, probably, the ovaries. From the base there arises also a little tenta cule or filament, susceptible of very great elongation, and which sends off many secon dary filaments.* The digestive organs of the ciliograda are less dubious. In these we find uniformly a straight alimentary canal with two orifices, the mouth inferior, the anus superior, in the ordi nary position of the animal. In some species there are lips formed by short and broad folds of the integument, four in number, and very sensitive. In cydippe, Dr. Grant found these lips capable of rapid extension and retraction.