38 



ACALEPII7E. 



animal's sinking into the deep when danger 

 threatens. In the other physograda, the air-ve- 



Fig. 7. 



sicle is so small in pro- 

 portion to the general 

 mass of the animal that 

 it is not sufficient to 

 raise it above the sur- 

 face of the water. It 

 is generally an ovate 

 sac, with an opening 

 at its upper end, closed 

 by a sphincter muscle. 

 It is probable that its 

 walls are muscular, and 

 that by pressing out a 

 portion of the contained 

 air, and by secreting 

 more, alternately the 

 animal can sink and 

 rise at pleasure. The 

 nature of the air con- 

 tained in these vesicles 

 has not yet been ascer- 

 tained. In rhizophysa 

 ( Fig. 7.) there are, pen- 

 dent from one part of the 

 body, certain peculiar 

 organs, arranged very re- 

 gularly in pairs, of a mus- 

 cular structure, hollow, 

 and furnished each with 

 a round orifice. They 

 differ from the tentacula 

 in structure, and are, pro- 

 bably, organs of natation, i 

 Similar tubes, but only / 

 two in number, exist in diphysa ; and, anterior 



Rhizophysa Melon. 



ings. Their attachment is so slight as to admit 

 of their being separated by agitation of the 

 water. It is at the bottom of the anterior ca- 

 vity that the essential parts of the animal 

 are placed. Locomotion is effected by means 

 of the impulse of a current which is kept up 

 by the successive contractions and dilatations of 

 the organs above described. The contractions 

 of the two bodies are not synchronous; but 

 they succeed one another within a short time, 

 so that a steady progression is maintained ; 

 and in some species iUs very rapid. 



In the ciliograda, the locomotive organs are 

 large cilia, disposed in longitudinal bands on 

 the surface of the body. These bands are ge- 

 nerally eight in number ; but in some species, 

 (e. g. uxiotoma Gaedii, Esch ,) there are only 

 four. The arches supporting the cilia are of 

 firmer texture, and are less transparent than 

 the rest of the body. In many species they 

 extend from one end of the body to the other ; 

 in some only along a part of the circumference. 

 The structure of the cilia themselves has lately 

 Fig. 9. 



Fig. 8. 



Agalma okenii. 



to them, in the same animal, 

 there is a two-lobed organ, 

 the use of which is doubt- 

 ful. In agalma, (Jig. 8.) and 

 some of the genera allied to 

 it, there are certain cartila- 

 ginous plates disposed in an 

 imbricated manner along the 

 sides of the body. These, 

 Eschscholtz regards as loco- 

 motive organs. The mus- 

 cles by which they are set in 

 motion must be extremely 

 delicate, as a slight touch is 

 sufficient to separate the 

 plates from one another. 



The chief bulk of the sin- 

 gularly formed diphyda is 

 made up of the swimming 

 organs, which are two sub- 

 cartilaginous bodies, poly- 

 gonal, generally pointed an- 

 teriorly, truncated posteri- 

 orly, placed one behind the 

 other, and one a little within 

 the other; the posterior por- 

 tion being lodged in a little 

 excavation which exists in 

 the anterior. These two parts 

 differ somewhat from one 

 another in form, but both are 

 hollow, and have large open- 



Diphyes Campanulifera. 



been examined by Dr. Grant,* with his usual 

 care, in the Beroe pileu&;-\ and he has found 

 that they are fin-like processes, and that each 

 is composed of several short, transparent, some- 

 what curved filaments, placed parallel to each 

 other in a single row, and connected together 

 by the skin of the animal, like the rays sup- 

 porting the fin of a fish. The rays in the 

 middle of the cilium are a little longer than 

 those at the sides. All the rays appear as 

 transparent tubes under high magnifying pow- 

 ers. They are so curved that their extremities 

 are directed backwards towards the closed ex- 

 tremity of the animal. There are about forty 

 cilia attached to each arch in this species, which 

 is nearly an inch in length. The cilia are so 

 large as to be visible to the naked eye. Most 

 of the ciliograda have their cilia quite exposed; 

 but Pandora is provided with moveable folds 

 of the skin along the cilia-bearing arches, which 

 can be brought over the cilia, in whole or in 

 part, at the animal's pleasure, so as to cover 

 them more or less completely. These cilia are 

 moved nearly in the same manner as the pec- 

 toral fins of fishes. But their motion is so 

 rapid, when the animal is vigorous, that the 

 eye cannot follow it. The existence of motion 

 is pointed out, however, by lines of beautiful 

 iridiscent colours playing along the arches, and 



* Trans. Zool. Soc. of London, i. 10. 

 t Pleurobrachia pileus. Fleming. Brit. Anim. 

 504. Cydippe p. Esch. 



