38 ACALErilS AS A CLASS. Part I. 



iiffinitv to Salainandoi's ami lo Ichtluoul-Batracliians could never have 1)een de- 

 termined with the (saiiie })i'eei8ion ; 1jut for our knowledge of the development of 

 the Comatuhv, that fiimily w(juld for ever have remained associated with the Star- 

 fishes ; and it seems to me that the inference is mia voidable, that the various 

 modes oi' develojmient of the Acalephs, as far as their embryology has already 

 been traced, must afford the surest clue to tlie natural affinities of these animals, 

 and, i)erha|)S, furnish a standard also b^' wliich we may determine to Avhat group 

 certain j'olyp-like Eadiata, alternately placed among Polyps and among Acalephs, 

 truly l)elong. Should their special homologies coincide with the indications fur- 

 nished by their endjryology, all doubts on this point would seem to Ijc removed ; 

 for, if the conclusions arri\ed at in those types of the animal kingdom which are 

 now l>est known lia\e any analogy with the phenomena observed in other types, 

 we should Ije able to trace sjtecial homologies l^etween all the representatives of 

 the class of Acalephs, in the same manner as Ijctweeu all Insects, or between all 

 Mammals. 



In this way, it would scarcely seem difficult to determine whether those ani- 

 mals which have been at diffiL»rent times referred to the class of Acalephs and to 

 that of Polyps truly belong to the one or the other, if the Polyps and Acalephs 

 indeed constitute two classes, or if not, to demonstrate satisfactorily that they 

 should form but one class. Again, all the representatives of the different classes 

 of one branch are found to agree in their general homologies, as far as they have 

 been thoroughly studied, — the Fishes with the Reptiles, Birds, and Mammals ; the 

 Insects with the Crustaceans and Worms ; and the Acephala with the Gasteropods 

 and Cephalopods. On the other hand, should there 1)e any animals, thus far re- 

 ferred to the class of Acalephs or to that of Polyps, which do not agree in their 

 general homologies with the true Polyps and the true Acalephs and Echinoderms, 

 we should not hesitate to remove them from the type of Radiates. Thus we may 

 also settle the question, whether the Ctenophora; are true Radiates or Mollusks, as 

 Quoy and Vogt have maintained. 



In order to avoid any hasty conclusions, let ns examine successi\'ely all the 

 leading representatives of every group that may have been associated witli either 

 the Acalephs or the Polyps, Ijoth Avith reference to their homologies and their mode 

 of development. Beginning with the Medusaj proper (PI. III., IV., V., VI., VII., 

 VIII., IX., XII., XIII., and XIV.), avc find them to l)e aniuuds Avhich move freely, 

 presenting an hemispheric gelatinous disk, in the centre of Avhich a digestive sac is 

 hollowed out. From the margin hang numerous filaments, and the central opening 

 is surrounded by four larger appendages. From the central cavity arise many tubes 

 radiating towards the periphery, Avhere they anastomoze. The essential feature of 

 this structure consists in the central cavity hoUoAved out of a continuous mass, 



