66 



DISCOPHOR^. 



Part III. 



Fig. 2. 



.'-'w'v. 



to the projecting edges of the radiating partitions, and in Echinoderms they rest 

 upon the interambulacral zones, either as a compact mass or in two rows, one on 

 each side of adjoining ambulacra. In the naked-eyed Medusae the same arrangement 

 obtains throughout, whether the sexual organs are situated along the chymiferous 

 tubes or upon the proboscis ; for in both cases these organs are upon the sides 

 of the medial channel of the ambulacral system, which is tantamount to occujjying 

 an interambulacral position. 



Now is it probable that the covered-eyed Medusse should alone form an exce23tion 

 to the plan of structure which obtains in all the Radiates ? Such exceptions exist 



• nowhere in the animal kingdom ; and if there 

 is any difficulty here, it can only be in the 

 interpretation of the facts, and in the con- 

 struction thus far jjut upon them. Let us 

 therefore examine Avhat the facts of the case 

 are. It has already been shown, page 52, that 

 the radiating chymiferous tubes of Aurelia {Fiff. 

 2) have not all the same origin, and that while 

 four systems of them communicate directly with 

 the main cavity of the body, four other sys- 

 tems, alternating with the former, arise from the 

 sexual pouches of which they are a direct con- 

 tinuation, as the others are a direct continu- 

 ation of the digestive cavity. 



The chief difference, then, between Aurelia 

 and the Hydroid-Medusse, consists in the greater isolation of the sexual organs from 

 the main chymiferous tubes ; but this separation is precisely in accordance with the 

 general progress of the organization of the Radiates, from the lowest Polyps to the 

 highest Echinoderms. In Polyps the ambulacra are wide chambers, and the inter- 

 ambulacra narrow partitions, upon the edges of which the sexual organs are inserted ; 

 in the naked-eyed Medusse the interambulacral system has become wide, and the am- 

 bulacral system is reduced to narrow tubes, but the sexual organs are still in the 

 immediate proximity of the chymiferous tubes ; in the Echinoderms, in which these 

 organs have become entirely independent of the ambulacral system, they are placed 

 in the middle of the interambulacral zones. In the Discophorte proper, they present 

 an intermediate combination ; separated from the four systems of chymiferous tubes 

 which arise from the main cavity of the body, they are connected with special 

 systems of chymiferous tubes, no longer directly opening into the main cavity, but 

 arising from the wide pouches in which the sexual organs are suspended. The cir- 

 cumstance that there is an eye at the peripheric termination of each median tube of 



AUKEUA FLAVIDULA, Per. <f LeS. 



c oral aperture. — oo genital organs. — mmm oral ap' 

 pendages, in outlines. — ete eyes 



-t tentacles. 



