136 DISCOPHOR^. Part III. 



a very scanty account of their structure, and no species of this family have thus 

 far been found within reach of sedentary observers, the only ones known being 

 those described by Peron and LeSueur. 



The largest number of species belonging to the family of RhizostomidiB, are for- 

 eign to the shores where observers could investigate them with the degree of care 

 and precision which, of late, has been bestowed upon all MedustB inhabiting the seas 

 of Eurojie and North America. It has, however, appeared to me very desirable 

 to compare all these species with ours, as far as the materials on hand would 

 permit, and to revise their arrangement in the light of our jjresent knowledge of 

 the Acalephs. In order to derive as much information as possible from these 

 materials, I have read, over and over again, every description, and compared every 

 figure relating to these animals, which has been published since the days of Pallas 

 and Forskal, weighing every word and trying to find out its true meaning. I 

 feel confident that I have in this way acquired an acquaintance with these Medusae, 

 and arrived at a knowledge of their true relations, more full and more accurate 

 than the observers who described them seem themselves to have possessed. I have, 

 therefore, ventured to express, in another chapter, the results of these comparisons, 

 in the shape of a tabular view, in the hope of presenting, as far as practicable, 

 a complete systematic review of all the Medusae known at 2:)resent, and also of 

 showing what may be done by a careful comjiarative study of old, and apparently 

 antiquated, materials. 



If I have read these data aright, the Ehizostomidje are not simply a family 

 among the other Discophora^, but constitute a distinct structural type among them, 

 of equal imjDortance and value as the other Phanerogamous Discophoraj of Eschscholtz. 

 This type appears to me to have the value of a sub-order, inasmuch as it shares 

 the general complication of its structure with Aurelia, Pelagia, Cyanea, and other 

 Discophoros, while it differs from them in such structural complications as affect only 

 the organization of some of its parts. These differences consist chiefly in the absence 

 of marginal tentacles along the edge of the disk, though the eyes are present, and 

 in the structure of the arms, the margins of which are soldered together, for a 

 greater or less extent, leaving only minute holes or short fissures along their edge, 

 which communicate with the main digestive cavity. The structure of the lower 

 floor, the formation and connection of the arms Avith that floor, the structure of 

 the genital pouches, the ramification of the main cavity in radiating chymiferous 

 channels extending to the margin of the disk, the structure of the eyes, in fact 

 all the leading structural features of these Acalephs are the same as in the other 

 Discophora3 ; they belong, therefore, to one and the same order. But as they differ 

 greatly in form among themselves, they constitute a number of distinct families, 

 which I have attempted to characterize in the next chapter, under the names of 



