Scientific Reviews. ^)9 



h\xt, in the geniis Pandora, tbey can be inclosed by means of folds 

 of the skin. The animal has also the power of moving either the 

 whole or only a part of them ; in the latter case, the body usually 

 acquires a rotatory motion. The fins, when separated from the 

 body, still continue to move briskly for some time. While in mo- 

 tion, the different rows of fins display the most beautiful iridescent 

 colours, with metallic lustre. The stimulus by which they are set 

 'm. motion, depends probably on a system of vessels similar to that 

 described by Tiedemann in the Echinodermata ; for there runs a 

 vessel under each row of fins." p. 3, 4. These last-mentioned ves- 

 gels we are inclined to view in a totally different light ; which opi- 

 nion, indeed, we shall find expressed by the author himself, a little 

 farther on, when he speaks of the respiratory organs. 



In the Acalepha of the two remaining orders, the organs of loco- 

 motion are entirely different. The Discophoree move onwards by 

 suddenly contracting their discoid or bell-shaped body against the 

 water ; and in the Syphonophorse the process is somewhat similar, 

 the animal being furnished with one or more hollow contractile or- 

 gans, which act on the water in the same way as the general mass 

 of the body in the Discophorse. 



• Among the passive organs of locomotion the author reckons the 

 air-bladder of the Physophoridae, and the cartilaginous or calca- 

 reous body, filled with air cells, which is found in the inside of the 

 animal in Rataria, Velella, and Porpita. The Discophoree, or Me- 

 dusae proper, are described by many observers as possessing air 

 vesicles, by which they are supported in the water, of which cir- 

 cumstance Peron was so convinced, that he assumed it as a founda- 

 tion for his arrangement of these animals. Mr. Eschscholtz, how- 

 ever, informs us that these supposed air vesicles are in reality not 

 so, the appearance of them being occasioned by air entering into 

 the sacs adjoining the stomach, to replace the water which escapes 

 when the animal is taken out of its native element. To the pas- 

 sive organs of locomotion also belongs the cartilaginous sail in the 

 Velella, by means of which the animal is carried by the winds along 

 the surface of the water. 



The next point of interest to which we shall advert, is the circul- 

 ating system in the Acalepha. No distinct vessels have as yet been 

 detected in the Discophorae ; but, in the animals of this order, the 

 digestive cavities are of great extent, and often ramified like ves- 

 sels through their surface ; which structure may probably compen?- 

 sate for the want of a more perfect circulating system. Most of 

 the Syphonophorae have distinct vessels, which spring from the 

 roots of the tentacula and the absorbing tubes, and spread out into 

 branches on the internal surface of the cavities, by means of which, 

 as already noticed, these animals propel themselves through the 

 .water. The last-mentioned organs, therefore, would appear to 

 serve also for respiration, since, while the animal is in motion, the 

 fluids in the vessels, distributed on their parietes, must be conti- 



