Chap. II. THE CLASSES OF RADIATA. 65 



The question, however, now arises, whether all these radiated animals form a type 

 distinct from the Echinoderms, as Leuckart would have it; or constitute two classes 

 of the type of Eadiata, coequal with the Echinoderms, as Cuvier represented them; 

 or three classes, as Owen and Ehrenberg admit; or any other number of classes. 



In uniting the Acalephs and Polyps into one primary division distinct from the 

 Echinoderms, Leuckart has overlooked the general homologies which unite the 

 Echinoderms with the Acalephs and the Polyps, and has paid no attention to the 

 Acalephian character of the embryo of a large numl)er of Echinoderms. There 

 is no feature more striking in all these animals, in the Polyps and Acalephs on 

 the one side and the Echinoderms on the other, than the radiated arrangement of 

 their parts. A comparison of Echinarachnius with Polyclonia and JEquorea, and of 

 the latter with Actinia, can leave no doubt upon this question ; and since all 

 Polyps can easily be reduced to the type of Actinia, as well as all Acalephs to 

 that of uEquorea and all Echinoderms to that of Echinarachnius or of Asterias, 

 it must be admitted that the plan of structure is the same in all these animals. 

 They are built upon the idea of radiation; that is to say, all their organs are 

 arranged around a centre, at which the mouth is placed, and diverge towards the 

 perijjhery, to converge again at an opposite pole. But this is not the whole : all the 

 organs of this structure are homologous. The chambers between the radiating 

 partitions of the Actinia correspond to the radiating tubes of iEquorea, and these, 

 again, to the ambulaeral system of the Echinoderms ; and the marginal tentacles of 

 the Actiniffi correspond to the marginal tentacles of the Acalej^hs, and appear as 

 ambulaeral tubes in the Echinoderms, under the various forms of seeming gills 

 around the mouth of Echinoids, or of seeming gills in the rosette of Clypeaster, 

 or of branching tentacles and ambulaeral suckers in the Holothurians. The iden- 

 tity of all these parts I shall have an opportunity of showing hereafter. 



The central cavity, in open communication with the radiating chambers in 

 Polyps, is closed in Acalephs, and communicates only through narrow openings with 

 the radiating tulles ; while in Echinoderms there arises a distinct alimentary canal, 

 which is, however, still in direct communication with the ambulaeral system through 

 a network of anastomoses, about which I shall also have more to say hereafter. 

 The ocelli at the base of the tentacles, which in Polyps are mere pigment cells, 

 appear like modified tentacles in the higher Medusa?, while they are still connected 

 w^th real tentacles in the lower ones ; in Echinoderms they appear again, in the 

 same relation with the ambulaeral system and the terminal odd ambulaeral sucker, 

 as they are with the tentacles in Acalephs. The sexual organs are upon the sides 

 of the radiating cavities; that is, upon the edge of the partitions in the Polyps, 

 upon the sides of the radiating tubes in the Acalephs, and alternating with the 

 ambulacra in Echinodenns, — everywhere in a homologous position and relation. 



