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or Crinoids, that in reading their descriptions of Starfishes it would 

 hardly appear that they belong to the same class. Echinoderms 

 thus far have been described by means of an entirely arbitrary termi- 

 nology ; there is no uniformity of language in the description of 

 Echinoderms belonging to diiFerent orders, and in the terminology 

 used for Crinoids there is such a difference, that it would seem as if 

 they had nothing in common with the other orders. It is with refer- 

 ence to this difficulty that I propose to make a few general remarks 

 — not that I mean to introduce a radical change, but only to modify 

 the language commonly used in such a way that its adaptation to all 

 the members of the class shall be easy. 



In the description of Starfishes we find that difierent authors, when 

 alluding to parts along the margins of the furrows on the lower sur- 

 face of the rays, speak of avenues which have been called ambula- 

 cral avenues, and of other parts as spines, which have been called 

 ambulacral papillae, implying a certain homology with the Echini ; 

 and thus far similarity is recognized between the ambulacral tubes of 

 the two orders. But a closer examination shows us that there is a 

 great diversity among the plates bordering on the ambulacral furrows 

 and among the spines articulating upon their surface, which require 

 to be distinguished with greater precision than has thus far been the 

 case. In reality, the plates and sj^ines bordering upon the ambula- 

 cral furrows belong to two different systems. To compare Starfishes 

 and Sea-urchins homologically, we must take into account their dif- 

 ferences as orders ; both have two distinct regions, one of which has 

 the mouth for its centre, from which radiate the ambulacra and 

 interambulacra, while in the other, opposite to it, are situated the 

 ocular and ovarian plates, and many other small parts. These two 

 areas I have called the actinal and abactinal areas, and it seems to me 

 that there is great advantage in dividing the animal at once in such 

 a way as to contrast strongly the parts of the ojiposite areas. Take 

 the case of Echini, composed as they are of ambulacral and interam- 

 bulacral zones. The actinal portion, consisting of these parts, makes 

 up nearly the whole surface of the animal, and the abactinal portion 

 is limited to a small area, more or less directly opposite to the mouth. 

 In Starfishes, on the contrary, these areas have about the same 

 extent ; the actinal area is generally limited to the comparatively 

 small space occupied by the mouth and the furrow on the lower side 

 of the rays, while the abactinal area occupies the interval between 

 the rays and the whole of the upper surface. These areas in Star- 

 fishes and Sea-urchins are homologous, notwithstanding this great 

 difference in their extent, which constitutes their essential distinc- 

 tion as orders, since it can be demonstrated that the whole back 

 of a Starfish and the sides of the ravs are homoloaus to the abactinal 



