ACALEPH^E. 



39 



by the currents which are generated in the cir- 

 cumambient fluid. The animal has the power 

 of arresting completely the motion of one, two, 

 or more rows of cilia, while the others are 

 moving. \\ hen all are set in motion together, 

 the animal moves onwards with the inferior or 

 oral surface (inferior in a state of rest) directed 

 forwards. When the motion of some is ar- 

 rested, the whole body acquires a rotatory mo- 

 tion, and advances in a curvilinear path. The 

 animal has also the power of changing the 

 direction of the currents caused by its cilia, so 

 that it can ascend or descend in the water at 

 pleasure. It can also increase and diminish at 

 will the velocity of the motions of the cilia. 

 Those animals which have the largest cilia, (e. g. 

 Medea,} swim with the greatest rapidity. The 

 cilia continue to move for some time after 

 having been separated from the body, in con- 

 nexion with part of their arches. Immediately 

 beneath the arches there are vessels conveying 

 a fluid, which is in motion during the vibrations 

 of the cilia. Whether these vessels are destined 

 only for the conveyance of the circulating fluid 

 to the cilia, (which in all probabi'ity act as 

 organs of respiration as well as of locomotion,) 

 or carry a stimulus fitted to excite their vibra- 

 tions, is not yet determined. Eschscholtz com- 

 pares these vessels to those which Tiedemann 

 has described as connected with the feet in the 

 echinodermata. And Dr. Grant is of opinion, 

 with MM. Audouin and Milne Edwards, 

 that it is not improbable that the motions of the 

 cilia are somehow dependent on the movements 

 of the fluids contained in the above-mentioned 

 vessels, seeing that in the actinia the tentacula 

 are projected by water being forced from below 

 into them. In the other classes of the acalephse 

 also the same kind of structure prevails. Such 

 of the pulmograda as have cilia around their 

 margins have also circular vessels running along 

 their bases; and almost all projectile and exten- 

 sile tentacules and filaments are provided with 

 sacs and canals, containing fluids, at their roots. 

 In addition to their cilia, several of the cilio- 

 grade acalephse have other organs of locomo- 

 tion in the form of long filamentary arms or ten- 

 tacules, with which they can poise themselves 

 in the water without moving their cilia. In Cy- 

 dippe* these are two in number. They are 

 lodged in two tubes placed alongside of the sto- 

 mach, from which they issue near the mouth. 

 They can be extended to four times the length 

 of the animal. They terminate in very fine 

 points, and along their whole course present 

 minute filaments placed at equal distances, 

 which are coiled up spirally, close to the ten- 

 tacules, when these are about to be withdrawn 

 into their sheaths. The tentacules are also 

 coiled up in a spiral form when completely 

 contracted. They are sometimes suddenly sent 

 forth from their tubes to their full length by 

 one impulse, and then their lateral filaments 

 are gradually uncoiled ; a process this of no less 

 interest on account of the gracefulness of the 

 motion than on account of the peculiar mecha- 

 nism which it indicates. 



* Grant. Trans. Zool. Soc. i. 10. 



The principal organ of motion in the pulmo- 

 grada is the large campanulate, or mushroom- 

 shaped, disc, of gelatinous consistence, which 

 constitutes the great mass of the animal. In 

 this, for the most part, no muscular fibres can 

 be seen, and yet the animals move about with 

 some quickness. They have the power of con- 

 tracting and dilating their discs at pleasure, in 

 whole or in part. By alternately contracting 

 and dilating their inferior surface, they strike 

 the water in such a manner and with such force 

 as to raise themselves ; when they discontinue 

 this motion, they again sink, being of greater 

 specific gravity than the sea-water. They move 

 onwards horizontally, by acting only with one 

 side of the margin of their disc. Lamarck 

 was of opinion that these isochronous move- 

 ments of the disc, by means of which the pul- 

 mograda seem to swim, were fitted merely to 

 facilitate the internal vital processes, and not 

 to move the animals through the water ; and 

 he regarded them as dependent entirely on the 

 influence of imponderable agents existing in 

 the circumambient fluid, and alternately enter- 

 ing into, and flowing from, the general mass of 

 the animal. He compared the motions with 

 those of the fluid in Franklin's thermoscope, 

 when held in the hand.* In the course of the 

 ordinary progression of the large Medusa aurita 

 of our seas, the contractions of the disc take 

 place from twelve to fifteen times in a minute. 

 The convex surface of the disc always advances 

 foremost. 



No fibrous structure has hitherto been dis- 

 covered in the general mass of the disc. In- 

 ternally, it is cellular, uniform, and very soft. 

 The quantity of solid matter in the disc, and, 

 indeed, in the whole body, is very small. 

 Some niediifxe, which, when recently taken out 

 of the water, weighed fifty ounces, on being 

 dried, left remains weighing scarcely more than 

 five or six grains. " It is therefore evident, 

 that the sea-water, penetrating the organic tex- 

 ture, constitutes the greater part of the volume 

 of these animals." f But in some species there 

 exists a fine muscular membrane, stretched 

 over a certain extent of the lower surface just 

 within its outer margin. Under a lens, this 

 has the appearance of being composed of nu- 

 merous fleshy fibres, forming little bundles, 

 arranged in a radiate manner as regards the 

 axis of the animal, and closely adherent to the 

 gelatinous tissue of the disc. When portions 

 of the disc are cut off from living medusa', 

 without any part of this muscular membrane 

 being attached to them, they remain motionless; 

 but when their connexion with the membrane 

 is preserved, even small portions continue their 

 motions of contraction and dilatation for a 

 considerable time. 



The tentacula of the pulmograda (which are 

 always pendent from the inferior surface) may 

 be regarded as supplementary organs of loco- 

 motion, although they are, in all probability, 

 subservient chiefly to the nutritive function. 

 They are all simple, not branched, generally 



218. 



* Anim. sans Vert. ii. 454. 



t Spallanzani, Travels in the Two Sicilies, iv 



