Chap. V. THE RHIZOSTOME^ IN GENERAL. 137 



RhizostomidfB proper, LeptobrachidiB, Cassiopeidte, Cepheidae, PolyclonidjB, and Favo- 

 nidse, all of which are held together, as a sub-order, by the structural peculiarities 

 mentioned above. Some of these families have already been pointed out as natural 

 groups by Tilesius, in his interesting paper on Cassiope^e, published in the Nova 

 Acta AcademisE Naturae Curiosorum, Vol. XV. In this paper the learned author 

 makes, however, several statements which cannot be correct, and must be distrusted 

 by every one familiar with the structure of the Acalephs. He states, for instance, 

 that water is expelled through the eight respiratory ventricles ; but what he calls 

 respiratory ventricles are the closed sacs formed by the genital pouches, which 

 have no communication whatsoever with the main cavity of the body of these 

 animals. Water, therefore, can only fill these cavities, and be moved in and out 

 by the contractions and expansions of the genital pouches, which shut the cavities 

 below them from all communication with the main cavity. He also aifirms that 

 a luminous gas is exhaled from the decomposed water, thi'ough the eight branchial 

 tubes and the marginal vesicles. I suppose that, under the name of marginal 

 vesicles, he alludes to the eyes, but I am at a loss to see how they can, in any 

 way, contribute to the decomposition of the water and the emission of a lumi- 

 nous gas. 



Our remarks upon the polystomy of the Rhizostomes lead, naturally, to some 

 further considerations upon the opinions which have, at different times, been ex- 

 pressed, with respect to the position and the absence of the oral aperture among 

 Acalephs. Peron and LeSueur have, in their classification of these animals, one 

 division which they call " Agastriques," some of which have been called " Astomes " 

 by Cuvier, and which they suppose to have neither central cavity, nor mouth, nor 

 peduncle, nor tentacles. In modern times, no Medusae have been observed exhib- 

 iting such characteristics. The genera referred to this division by Peron and 

 LeSueur were, no doubt, founded upon imperfect specimens. The others, which 

 are called " Gastriques," are divided into Monostomes and Polystomes ; the Polystomes 

 being all those which have distinct genital sacs, inserted above large openings of the 

 lower floor, formed by the thinning of that floor and its inversion into the main 

 cavity, or its eversion in the shape of a pendant sac below it. These , openings 

 Peron and LeSueur have mistaken for mouths, and they have overlooked, in some 

 of them, the real oral aperture. This is, for instance, the case in Aurelia, which 

 is characterized as having four mouths, by which can be meant only the four large 

 funnels below the genital sacs, while the mouth, between the four arms, has not 

 been observed by them. 



VOL. IV. 18 



