THE ECIIINODKU.MATA. 



45 



P 



Fig. 20. — Development of an Echinid larva (after Miiller). — A. earliest, and B, later 

 condition of larva. C. The Echinid imago developed within and nearly obliterating 

 the larva. 



After swimming about in this condition for a while, the 

 larva begins to show the first signs of those changes by which 

 it is converted into the adult Echinoderm. An involution of 

 the integument takes place upon one side of the dorsal region 

 of the body, so as to give rise to a csecal tube, which gradually 

 elongates inwards, and eventually reaches a mass of formative 

 matter, or blastema, aggregated upon one side of the stomach. 

 Within this, the end of the tube becomes converted into a 

 circular vessel, from which trunks pass off, radially, through the 

 enlarging blastema. The latter, gradually expanding, gives rise 

 in the Echiniclea, the Asteridea, the Ojrfiiuridea, and the Crinoidea, 

 to the body-wall of the adult; the larval body and skeleton 

 (when the latter exists), with more or less of the primitive 

 intestine, being either cast off as a whole, or disappearing, or 

 becoming incorporated with the secondary development, while 

 a new mouth is developed in the centre of the ring formed by 

 the circular vessel. The vessels which radiate from the latter 

 give off diverticula to communicate with the cavities of 

 numerous processes of the body — the so-called feet — which are 

 the chief locomotive organs of the adult. The radiating and 

 circular vessels, with all their appendages, constitute what is 

 known as the " ambulacral system ;" and, in Asterids and 

 Echinids, this remarkable system of vessels remains in commu- 

 nication with the exterior of the body by canals, connected with 

 perforated portions of the external skeleton — the so-called 

 " madreporic canals " and " tubercles." In Ophiurids the per- 

 sistence of any such communication of the ambulacral system 



