XIV INTRODUCTION. 



with the movements of the animals ; for it is by means of 

 this water circulation that the suckers or cirrhi are enabled 

 to act as organs of progression. In many species of the 

 most typical group, that of Echinidse, we find a portion of 

 the dermato-skeleton turned in, as it were, to form arches 

 for the protection of the water-canals, thus evidencing their 

 great importance in those creatures. Among the Annel- 

 lidous Echinodermata, however, the aquiferous system 

 seems altogether to disappear. 



On the modifications of this characteristic system, its 

 presence or absence, and its combination with the tegu- 

 mentary system for purposes of motion, I have founded my 

 arrangement of the Echinodermata. I look upon the Echi- 

 nodermata and Araclmodermata as two parallel groups, 

 and hold it as a law that the divisions of parallel groups 

 should he based on a common principle. The orders of the 

 latter class have always been founded on the modifications 

 of their organs and modes of progression : the orders of 

 the class before us I have founded on the same consider- 

 ation, and need only call the attention of the philosophical 

 zoologist to the naturalness of the divisions so formed, and 

 to their equidistance from each other, and I feel confident 

 he will acknowledge the truth of my arrangement. 



Order I. Pinnigrada. Crinoide^e — First appearance of cirrhi, 

 springing from brachial membranes, which, with the 

 true arms, form the organs of motion. 

 II. Spinigrada. Ophiurid;e — Disappearance of brachial 

 membranes, cirrhi as before ; true arms clothed with 

 spines for motion. 

 III. Cirrhigrada. Asteriad^; — Arms disappear ; body 

 more or less lobed, and lobes channeled beneath 

 for cirrhi, which act as suckers, and are the organs 

 of motion. 



