ACALEPH^E. 



37 



Body oval or circular, gelatinous, supported by 

 an internal, solid, subcartilaginous body, and 

 provided with very extensile tentacule-like 

 cirri pendent from the whole of the lower sur- 

 face. 



Gen. Velella. Porpita. Rataria.* 

 Of the genera above enumerated, Eschscholtz 

 has described about two hundred species. 

 Messrs. Quoy and Gaimard have made us ac 

 quainted with several others ; but of all these 

 a comparatively small number only have been 

 described in detail: so that, although in the 

 account which we are now to give of the 

 anatomy and physiology of the acalephae, we 

 shall, for the sake of brevity, make use of the 

 sectional designations, it must be understood 

 that the descriptions apply only to a few 

 species, and that, with regard to the others 

 grouped along with these, we can only say 

 it is probable that they are similarly con- 

 structed. 



I. Locomotion. The principal organ of 

 locomotion in the physograda is the air-filled 

 vesicle or bladder, which exists, of various 

 sizes, in all the species. Inphysalus, (Fig. 6.) 

 it is a large organ, forming a great portion of 

 the general mass of the animal. It is placed 



* The following neat but artificial arrangement 

 of acalephcB forms the subject of a communication 

 lately made to the Zoological Society by M. Lesson, 

 foreign member of that body : we are indebted for it 

 to our friend Mr. Owen. 



I. Without a central solid axis. 



A. Body simple, entire. 



1. Symmetrical, termi- 

 nated at each pole by 



an opening .... 1 Beroidce. 



2. Non-symmetrical, the 

 upper pole disciform or 

 umbelliform, imper- 



forate 2 Medusa;. 



B. Body multiple or aggre- 



gated. 



a. Homogeneous. 



3. Composed of two pieces 

 adhering together, and 



capable of separation . 3 Diphydcs. 



4. Composed of numerous 

 pieces aggregated toge- 

 ther 4 Polytoma. 



b. Heterogeneous. 



5. Animal furnished with 

 appendages of different 

 kinds. 



1 Vesicle small, regular, 

 placed at the summit of 

 a kind of stalk fur- 

 nished with lateral am- 

 pullae, and terminal 



suckers 5 Physsophorce. 



Vesicle large, irregular, 

 without stalk or am- 

 pullae, but having ter- 

 minal suckers and cir- 

 riferous processes . . 6 Physaliee. 

 II. With a central cartilaginous 

 axis. 



6. Body simple, with 

 suckers and lateral ten- 

 tacula. 



superiorly, and, for the most part, rises above 

 the surface of the water. It has an elongated 

 form ; the longest diameter being the hori- 

 zontal. It is somewhat pointed at one end, 

 at the other truncated ; and at either there is 

 a small opening, the place of which is marked 

 by a superficial dimple, surrounded by delicate 

 muscular fibres, acting as sphincters. When 



(Fig. 6.) 



a. Body irregularly oblong, 



with a vertical lamina 



on its upper surface . 7 VelellfB. 



b. Body discoid, flat above. 8 Porpitte. 



R. B. T. 



Physalus Utriculus, (Esch.) 



the bladder is squeezed by the hand, so as to 

 force the contained air towards one of these 

 openings, the air makes its escape through it; 

 but whenever the pressure is taken off, the 

 opening again closes. M. De Blainville states 

 that he has satisfied himself that this air-blad- 

 der is really a dilatation of the intestinal canal; 

 and that he regards the two openings mentioned 

 above as the mouth and the anus. We are 

 ignorant of the data upon which M. De Blain- 

 ville grounds his conclusion. It does not ap- 

 pear that any observer has found alimentary 

 matter lodged within the air-sac. But whether 

 or not it be an organ of digestion, it is cer- 

 tainly an organ of locomotion, although only 

 a passive one ; for it is by its contained air that 

 the animal floats on the surface of the water, 

 so as to expose a large superficies of its crest 

 and bladder to the wind, by which it is driven 

 to and fro frequently with great velocity. The 

 walls of this sac are muscular, so that by their 

 contraction its cavity can be considerably dimi- 

 nished. And thus, partly by the escape of air 

 forced out through the openings, and partly by 

 the compression of what remains, the specific 

 gravity is so much altered as to admit of the 



