66 ACALEniS IN GENERAL. Part I. 



Leaving a^ide, for the jjreseiit, some farther compUcation in the structure of Echi- 

 noderms, which we shall consider more fully in the latter part of this monognqjh, 

 it can, finally, be said, that the Echinoderms are Aealephoid animals, the body- 

 wall of which is loaded with limestone. Lamarck had truly perceived this close 

 affinity between the Acalephs and Echinoderms when he united them into one 

 great division under the name of Radiaires to the exclusion of the Polyps, calling 

 the Acalephs " Radiaires Mollasses," and the Echinoderms " Radiaires Echinodermes." 

 In tlnis closely combining these two types into one class, however, he committed 

 an error similar to tliat of Leuckart, who united the Polyps with the Acalephs 

 in one larger group, from which he excludes the Echinoderms. But the reference 

 already made to the liomology of their structure is in itself sufficient to show that 

 Poly^js, Acalephs, and Echinoderms are constructed upon the same plan, and ought 

 therefore to be iniited in one and tlie same primary division, for which tlie name 

 of Radiata, j'l'oposed ))y Cuvier, seems to Ije the most appropriate. This once 

 settled, the question of the sul)di vision of the Ra<liata into classes becomes com- 

 paratively easy. 



I take it for granted, tliat tlie distinction I have attempted to make ' between 

 the piiiii of struciiirc in animals and the mode of execution of tlie plan is, if not 

 admitted by other naturalists, at least fully understood by them ; and upon this 

 basis I now propose to discuss the limitation of the classes of Radiata. Admitting 

 the jiV«« of structure to be the criterion by which the primary groups of animals 

 are distinguished, we have seen that Echinoderms cannot l)e separated from the 

 other Radiates, since they differ only in structural complications, but not in the 

 plan of their structure. Admitting, next, that the mode of execidioii of a given 

 plan of structure constitutes the essential difference between classes, we ha\e now 

 to consider in what Avay the idea of radiation (upon which the plan of structure 

 of the Radiates is founded) is carried out in different types of this branch of the 

 animal kingdom, and with what means their body is Iniilt up ; and this Avill furnish 

 us with a key to find the natuial limits of their classes. 



The leading characteristics which distinguish the Polyps, the Acalephs, and the 

 Echinoderms, are so obvious that it is only necessary here to allude to their 

 most prominent features, in order to show that they are essentially different in 

 their anatomical structure, though built iipon the same ])lan. Li Polyps, the body 

 has the form of a sac, from the inner surface of which project radiating partitions, 

 leaving an open space in the centre, however, which is the main cavity of the 

 bod}'. This central cavity is in free eommunii'ation with the radiating chambers 

 enclosed between the radiating partitions, for the whole height of the body. In 



^ Sec Vol. 1 of this work, pp. 137 :iiul 14.'). 



