Chap. II. MORPHOLOGY AND NOMENCLATURE. 73 



SECTION IV. 



MORPHOLOGY AND NOMENCLATURE. 



Thus far, my aim lii^s^ heon to present an outline of the views entertained l)y 

 different naturahsts upon the various rehitions among tlie animals referred to the 

 type of Radiata, taking that group in the widest sense in which it has ever been 

 considered. I have accompanied this survey with incidental critical remarks, and 

 with a few considerations upon the mode of ascertaining the natural limits of a 

 class, and have arrived at the conclusion, that the type of Radiates embraces only 

 three natural classes. This conclusion is founded upon the evidence adduced, that 

 the animals heretofore referred to Radiates, and not belonging to the one or the 

 other of these three classes, are not genuine Radiates, and must therefore be ex- 

 cluded from that type. 



I have attempted to show, farther, that the proposed division of Radiates into 

 Coelenterata and Echinodermata, as distinct primary types, is a mistake arising 

 from an incorrect appreciation of what constitutes respectively a type or l)ranch, 

 and a class, in the animal kingdom. If the views I hold on this subject are true, 

 the Echinoderms, being built upon the same plan as the Polyps and Acalephs, 

 belono- to the same type as the so-called Coelenterata, and constitute only one 

 class of that type. The peculiarities insisted upon as a ground for considering 

 Echinoderms as a distinct type are not differences in the plan of structure, but 

 merely differences in the mode of execution of one and the same plan. 



I hold, farther, that the Coelenterata, as circumscribed by Leuckart, embrace 

 two distinct classes, the essential characters of which are of the same kind as those 

 that separate the Echinodenns from either of them; so that, considering classes to 

 be founded on different ways of carrying out the same structural plan, the type 

 of Radiata should "be divided into three classes, — the Polyps, the Acalephs, and the 

 Echinoderms. It is true that the range of structural differences in these classes, 

 within their respective limits, is not always exactly parallel; l)ut it is a fact, too 

 much overlooked by naturalists, that there are very few groups in nature of the 

 same essential value, presenting identical degrees of difference, or even approximating 

 each other in their number of genera and species. 



In the reo-ular sequence of my exposition I should now present a sketch of 

 the natural features of the class of Acalephs ; but before I make the attempt, a 

 few words upon their morphology and nomenclature are indispensable. This is 

 important, in order that I may be able to present the characteristics of the class 



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