PHYLUM ECHINODERMATA 



361 



tasis.org 



vessel of the ainbulacral system. A pore, the dorsal p <ore (dors, p), 

 es its appearance as an invaginatioii of the ectoderm nearly 



opposite the larval mouth ; this 

 opens internally into the left entero- 

 coelic pouch. A groove is formed on 

 ^ e inner surface of the hydrocoele, 

 and becomes closed to form a canal, 

 the rudiment of the madreporic 

 canal ; this subsequently enters into 

 communication with the dorsal pore, 

 the external opening of which corre- 

 sponds to the future madreporite. 



A change has meanwhile taken 

 place in the general form of the body 

 of the larva. At the upper end has 

 been formed an anterior short lobe 

 (anterior head-lobe) and a posterior 

 longer lobe (posterior head- lobe). 

 These two lobes together form what is termed the larval organ 

 (larv. org), which later on becomes absorbed in the transition to 

 the starfish form. As the hydrocoele develops, its form influences 

 the external shape of 



"": 



290. Bipimutriu of Asterina, 

 view of the left -hand side, showing 

 the tive-lobed prominence, aruli, 

 formed by the developing ambu- 

 lacral system on what is destined 

 to bec'>me the ventral surface of 

 the b,,dy of the Starfish. 



fen? 



t.f. 



the larva ; on the left- 

 hand side there grows 

 out a fiye-lobed eleva- 

 tion (Fig. 290, ami), 

 oach of the lobes cor- 

 responding to one of 

 the five lobes of the 

 hydrocoele. Each of 

 the latter then be- 

 comes divided, first 

 into three rounded 

 processes (Fig. 289, B, 

 amb), and then into 

 five, and these project 

 freely on the surface; 

 the middle one is the 

 rudiment of the ten- 

 tacle, the lateral pro- 

 cesses are the first two 

 pairs of tube-feet. At 

 the same time five 

 elevations of the op- 

 posite wall, which have become evident, give rise to the beginnings 

 of the dorsal regions oi the arms. 



The transition from the larval Bipihnaria stage, as it is termed, 



FIG. 291. Asterina exigua. Young Starfish shortly 

 after 'the metamorphosis has been completed, viewed 

 from the ventral side. circ. ami. circular ambulacral 

 vessel ; dors. p. dorsal pore and madreporic canal ; rad. 

 amlt. radial ambulacral vessel ; st. stomach ; tciit. tentacle ; 

 t. f. tube-feet. 



