L. Agassiz on the Echinodermata. 43 



two contiguous rays in the latter : for the pieces exhibiting its 

 growth are formed in the angles of the rays on the upper and 

 lower surface of the body, and, becoming larger and larger, 

 elongate the rays and drive the extremities of them to a greater 

 distance ; so that the number of the plates continually in- 

 creases and cannot be considered as a specific characteristic. 

 The growth of the summit in the Asteriae and of the stem in 

 the Crinoides, as well as that of the moveable pieces of the 

 mouth, is also independent of the rays in these animals, and 

 accompanied by a peculiar position of their parts, as in the 

 Echini. Hence it is easy to conceive how a body of the star 

 kind may grow larger and still retain its form. 



The study of the organization of the Echinodermata has led 

 me to introduce in their classification and in the defining of their 

 genera some changes, which I shall also exhibit in a synoptical 

 table. I found that the characters deduced from the combi- 

 nation of the plates and the arrangement of the ambulacra 

 formed groups more natural and better defined than those de- 

 duced from the position of the mouth and the anus. 



The class of the Echinodermata is divided into three orders: 

 the Stellerides, the Echinides, and the Fistulides, which re- 

 semble in the degree of their organization the three classes of 

 the Radiata. The Stellerides answer to the class of the Polypi ; 

 the Echinides to that of the Acalephae, which connect the sec- 

 tion of the Radiata with the Mollusca ; while the Fistulides ? as 

 the culminating point of this division, represent the section of 

 the articulated animals, and more especially the Vermes. 



As to the genera established in this class, I found that the 

 characters deduced from the combination of the plates and the 

 disposition of the ambulacra formed groups more natural and 

 better defined than those deduced from the situation of the 

 mouth and the anus. My observations on this subject I shall 

 publish in a monographic paper (accompanied with plates);, 

 for which I have already collected most of the necessary ma- 

 terials. 



[To be continued.] 



