AGASSIZ. NATURAL HISTORY OF T1IE UNITED STATES. 435 



the following paragraphs/ he more fully exhibits his views on the 

 same subject i — 



"In uniting the Acalephs and Polyps into one primary division 

 distinct from the Echinoderms, Leuckari has overlooked the general 



homologies uliicli unite the Echinoderms with the Acalephs and 

 Polyps, and has paid no attention to the Acalephian character of the 

 embryo of a large number 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 arrange- 

 ment of their parts. A comparison of Echinarachnius witli 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 JEquorea, and all Behi- 

 noderms to that of Echinarachnius or of Aster ias, it must be ad- 

 mitted 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 periphery, 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 JEquorea, and these, 

 again, to the ambulacral system of the Echinoderms ; and the mar- 

 ginal tentacles of the Actinia correspond to the marginal tentacles of 

 the Acalephs, and appear as ambulacral tubes in the Echinoderms, 

 under the various forms of seeming gills around the mouth of Echi- 

 noids, or of seeming gills in the rosette of Clypeaster, or of branch- 

 ing tentacles and ambulacral suckers in the Holothurians. The 

 identity 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 tubes ; while in Echino- 

 derms there arises a distinct alimentary canal, which is, however, 

 still in direct communication with the ambulacral 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 with real tentacles 

 in the lower ones; in Echinoderms they appear again, in the same 

 relation with the ambulacral system and the terminal odd ambu- 

 lacral 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 \x\{\\ the ambulacra 

 in Echinoderms, — everywhere in a homologous position and re- 

 lation." * 



* P. 65. 



