ACALEPH7E. 



41 



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 acalephae 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 acalephae, 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 acalephae 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 

 medusas, 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.f 



' Isis. Nov. 1819. 



t Professor Ehrenberg has very recently attempted 

 to shew that medusa aurita is possessed of eyes, in 

 the form of minute red points, which are seen on 

 the surface of the eight brown-coloured masses set 

 round the circumference of the disc. These masses, 

 according to his observations, consist each of a yel- 

 lowish, oval, or cylindrical little body, which is at- 

 tached to a small and delicate pedicle. This short 

 pedicle arises from a vesicle, in which there is 

 placed a glandular body, unattached, presenting a 

 yellow colour when viewed with transmitted 

 light, a white colour under reflected light. It 

 is upon the dorsal aspect of the yellow head, 

 which surmounts the pedicle, that the well denned 

 red point is seen, which Ehrenberg considers as an 

 eye. HP. compares the eyes of medusa to those of 

 some rotifera and entomostraca. The glandular body 

 situated at the base of the pedicle, he regards as an 

 optic ganglion, which, he seems to have satisfied 

 himself, is connected with two filaments that decus- 

 sate one another at about the middle of their course. 

 These he describes as forming part of a nervous 



III. Digestion. The structure and action 

 of the organs concerned in the function of 

 digestion in the acalephae 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. 



circle placed, throughout the greater part of its 

 course, immediately along the bases of the row of 

 tentacules that surround the disc, so as to form, as 

 it were, the outer wall of the circular vessel, or ap- 

 pendage of the intestinal cavity, which runs round 

 the margin of the disc. The same observer de- 

 scribes another nervous circle, composed of four 

 ganglion-like masses, disposed around the mouih, 

 each being in connexion with a corresponding group 

 of tentacules. (Ehrenberg, in Miiller's Archiv 

 fur Anat. Physiol., &c. 1834. p. 562.) 

 * Ann. des Sc. Nat. x. 8. 



