PLAN OF DEVELOPMENT OF ECHINODEKMS. 83 



If the views here taken of the phin of development of Echinoderms be 

 correct, they introduce a new set of facts respecting their affinities with 

 the Polyps and Acalephs, which cannot fail to have an important bearing 

 on the question of the separation of the Echinoderms as a distinct type 

 from the two latter groups. The Echinoderm plutean form, with its mouth, 

 stomach, intestine, and with its water-system originally forming a part of 

 the digestive cavity, bears, as it seems to me, the same relation to the 

 Ctenophoroe which the Hydroid Polyps hold to the true Polyps. Ihe Cte- 

 nophora? may be considered, as it were, the prototype of the Echinodenn.s, 

 as the Polyps are the prototype of Acalephs. We have in the Ctenophorno 

 a digestive cavity, from which branches the water-sj-stem, and that peculiar 

 funnel opening outwards, through which the fecal matters of the Cteno- 

 phorae are discharged, reminding us at once of the almost identical arrange- 

 ment of an Echinoderm Pluteus, in the relations of the intestine to the 

 stomach. The plutean forms certainly show that the plan upon Avhicli the 

 Echinoderms are built does not differ from that upon which the Acalephs 

 are built, and that we have between the Echinoderms and Acalephs the 

 same connection, based upon identity of plan, as exists between the Acalephs 

 and Polyps. We cannot, therefore, admit that the views so frequently 

 urged and so universally admitted, in support of the separation of the 

 Acalephs and Polyps as a distinct type (Ccelenterata), from the Echino- 

 derms, have any real foundation in nature ; and still less can we concur 

 in them when we remember that the main aro-ument in their favor rests 

 upon the assumed total want of connection between the ambulacral system 

 and the digestive system. This connection has been shown by Pro- 

 fessor Agassiz to exist in the adult of many Echinoderms, while the facts 

 above stated prove that it also exists in the early stages of the embryonic 

 development, where, in fact, the water-system is formed from the digestive 

 system. With this evidence falls the strongest argument for the validity 

 of a classification by which the type of Radiates would be broken up, and 

 the Polyps and Acalephs separated from the Echinoderms, as a dis- 

 tinct type, under the name of Ccelenterata. We are, therefore, justified 

 in affirming that the type of Radiates constitutes an independent type 

 of the animal kingdom, containing three equivalent classes, — Echinoderms, 

 Acalephs, and Polyps. 



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