PHYLUM ECHIXODERMATA 



361 



Last;, or p 



FIG. 290. Bipinnaria of Asterina, 



view of the left-hand side, showing- 

 the flve-lobed prominence, ami, 

 formed by the developing amlm- 

 laiTil system 011 what is destined 

 to become the ventral surface of 

 the bndv of the Starfish. 



vessel of the ambulacral system. A pore, the dorsal pore (dors.p), 

 makes its appearance as an invaginatioii of the ectoderm nearly 



opposite the larval mouth ; this 

 opens internally into the left entero- 

 ccelic pouch. A groove is formed on 

 the 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 licad-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 



the larva ; on the left- ent~ 



hand side there grows 

 out a five-lobed eleva- 

 tion (Fig. 290, w&), 

 each 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 of the arms. 



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



t./. 



ami) 



si 



FIG. 291. Asterina exigua. Young Starfish shortly 

 after the metamorphosis has been completed, viewed 

 from the ventral side. circ. amb. circular ambulacral 

 vessel ; dors. p. dorsal pore and madreporic canal ; <(/. 

 unit,, radial ambulacral vessel ; ft. stomach ; t< ,>t. tentacle ; 

 t. f. tube-feet. 



