ECHINODERMA TA. 169 



of the animal (presently to be described), called the 

 ambulacral tubes, or feet. The whole system is 

 filled with fluid, and by means of this the feet can be 

 inflated, at will, so that the animal can push them out 

 to walk with or draw them back when it is at rest. 

 The fluid in this system is indirectly in communication 

 with the sea-water outside, by means of a tube which 

 is called the stone-canal, and which ends exteriorly in 

 a structure called, from the curved pattern on it 

 (which is like the winding outlines of the madrepore, 

 or brain coral), the madreporiform tubercle. 

 The water-vascular system arises in the larva, as a 

 branch of the body-cavity. 



The Echinodermata used to be considered to be 

 radial animals like the CcBlenterata, and were included 

 by Cuvier in his Eadiata, along with these. But their 

 radial symmetry appears to be secondary, as well as 

 incomplete, for they are derived from a bilateral larva, 

 aa well as being irregular in the adult stage. Roughly 

 speaking, however, they have in the adult a penta- 

 merous symmetry [i.e. composed usually of five parts). 

 The true position of the group with regard to others 

 is very uncertain ; all that can be said positively is, 

 that its larval stages resemble those of such worms as 

 have a larval stage, so that we may suppose the two 

 groups of animals to be in some way connected. 



The Echinodermata are only placed before the Worms 

 in classification for the sake of convenience, because 

 nobody knows what place to give them ; for among 

 the varied groups that are put under the heading of 



