G8 ACALEPIIS IN GENERAL. Part I. 



in their structure with the true Medusae This agreement is complete ; and there 

 is no room left for a distinction between Ilydroids and Meduste, any more than 

 for a reunion of Polyps and Ilydroids. 



The essential structural peculiarity of the Acalephs, as a class, consists in the 

 presence of a central cavity, hollowed in tlie mass of the body, without radiating 

 partitions, but with an external central opening, the edge of which is turned out- 

 ward and more or less prolonged, in the shape of oral appendages or fringes. 

 Tentacular appendages may also exist outside of this central opening, or so-called 

 mouth, or may be wanting ; but when they do exist, their cavity, if they are 

 hollow, communicates only indirectly, through radiating tubes, with tlie main cavity 

 of the body, the radiating tubes themselves uniting Avith a circular tube that 

 follows the outline of the periphery. This is certainly an essentially different 

 structure from tliat of the Polyps. Again, while the Polyps are always sexual 

 animals, and frequently hermaphrodites in their adult age, the Ilydroids are uni- 

 formly destitute of sexual organs, but produce, by budding, an alternate generation, 

 the individuals of whicli, like ordinary Medusa^ are always, when adult, either male 

 or female. When considering in detail the structure and mode of reproduction 

 of the Acalephs, I shall have occasion fully and conclusively to show that the 

 parts generally considered as generative organs in the Ilydroids are truly indi- 

 vidual animals, in every way homologous to true Medusa, and themselves provided 

 with the sexual organs that are wanting in the Ilydroids. For the present I must 

 limit myself to the assertion that it is so. 



As to the homology between Polyps and Acalephs, it must be apparent, from 

 what precedes, that the comparisons which have been instituted between them are 

 not accurate. If the central opening between the tentacles of the Polyps is not 

 homologous to the so-called mouth of the Acalephs, but simply an aperture arising 

 from such an inversion of the body-wall that the opening at the bottom of the 

 digestive cavity is in reality the external opening of the body, it is plain that 

 the name nmith has been applied to very different parts in these animals. It 

 must further appear, that, from the position of this opening and its relation to the 

 whole structure of the animal, the name mmifh can hardly be applied to it. Indeed, 

 the more we study the lower animals, the more are we impressed with the imper- 

 fection of the nomenclature used to designate their parts. To me it now seems 

 quite inappropriate to designate the ojjening through which the food is introduced 

 into the body by the same name in all animals. Since the study of homologies 

 has become a safe guide in the appreciation of the true nature of the parts of 

 an anim;d, I can no longer see why we should use the name mouth to designate 

 a simple opening in the centre of a radiated structure, when that name was 

 originally applied to a cavity circumscribed by a bony frame, Avith a muscular 



