Chap. II. THE DIFFERENT RADIATA. 41 



SECTION II. 



THE DIFFERENT ANIMALS REFERRED TO THE TYPE OF RADIATA. 



I shall presently show that all the true Polyps and all the true Acalephs may 

 naturally be grouped with the two characteristic representatives of their respective 

 classes, alluded to in the preceding section ; and that, in connection with the Echi- 

 noderms, they constitute one of the four great types of the animal kingdom, 

 characterized by a pecidiar plan of their structure, founded upon the idea of radi- 

 ation ; and that the anatomical differences exhibited by the Echinoderms do not 

 justify us in considering them as a distinct type.^ The latter are, in reality, only 

 another class of Eadiata, as a comparison of any of the flat Eehinoids, such as 

 the Echinarrachnius, with an ordinary Medusa, say the Aurelia, readily shows; 

 Echinus being, as it were, a Medusa, the soft disk of which is charged with lime- 

 stone particles. But before proceeding to demonstrate these propositions, it is 

 proper to take a glance singly at all the different beings which, at different times, 

 have been associated with or removed from the Eadiata. 



Whether considered as a distinct type, or simply as a class of the Radiata, 

 the Echinoderms, as a natural group, are now very generally circumscribed within 

 the same limits by all naturalists. The question, long agitated among zoologists, 

 whether the Sipunculoids should be associated with the true Echinoderms or referred 

 to the class of Worms, has finally been settled in flxvor of their complete removal, 

 by the investigations of the late lamented J. Muller.^ We may henceforth con- 

 sider as Echinoderms all the radiated animals provided with an amljulacral system, 

 and need not for the present enter into a farther consideration of their structure 

 and i--eneral affinities, but leave them out of consideration until we attempt to trace 

 the general homologies, which, in connection with their mode of development, bind 

 these animals indissolubly with the Acalephs and Polyps as a separate class of the 

 type of Radiata. 



The natural limits of the class of Acalephs cannot be considered as settled, 



1 The separation of the Echhioderms from the types seems unjustifiable, since the consideration of 

 otlier Radiates, as a distinct type, was first pro- tlie complication of their structure is surely a feat- 

 posed by Leuckart in the work quoted on the ure subordinate to the idea of their plan of struct- 

 preceding pa;;e. This distinction has been adopted ure; and the mode of execution of a plan should 

 by Kolliker, and by Gegenbaucr in his recent excel- not be confounded with the plan itself, 

 lent text-book of Comparative Anatomy. To me, - Ueber den Ban der Echinodermcn, Ak. d. 

 however, such a division of the Radiates into two Wiss. Berlin, Ibo-l. 



VOL. III. 6 



