DEVELOPMENT OF OPHIURIDS AND HOLOTHUBIANS. 



151 



tioned that there is a well-developed nervous system in the larva of 

 Echinus. It has the form of an apical plate of neuro-epithelium, placed 

 on the preoral portion of the body dorsal to the ciliated band and between 

 the preoral arms. It is not recognizable till the larva is three weeks old. 



In the metamorphosis the animal falls to the bottom, the amnion rup- 

 tures and shrivels up, the larval arms are absorbed by phagocytic amoe- 

 bocytes, the larval mouth and anus close and the somewhat spherical 

 form of the adult is assumed. At first the metamorphosed animal is with- 

 out a mouth and anus ; these soon appear, the mouth first. The anus 

 is formed in the centre of the antambulacral surface which is at first equal 

 in area to the ambulacral. 



In Ophiurids a single coelomic sac is budded off from the anterior end 

 of the archenteron. This soon divides into a right and left sac. Each 

 of these divides into anterior and posterior coeloms. The hydrocoels 



FIG. 107. Longitudinal vertical sections of the two stages of the larva of Synapta digitata 

 showing the formation of the enterocoel and of the primary water-pore P. Bl blastopore 

 (after Selenka). 



then arise from the hind end of the anterior coelom in the usual way 

 (MacBride) and the left anterior coelom develops a water-pore. The 

 left hydrocoel alone acquires lobes and develops into the water-vascular 

 system. As in Asterina the rudimentary right hydrocoel in abnormal 

 specimens occasionally acquires a form similar to the left. 



In Holothurians the single pouch acquires an opening to the exterior by 

 a pore placed on the dorsal surface and just to the left of the middle line. 

 This is the primary water-pore. In Synapta it may even be formed before 

 the enterocoel has separated from the enteron (Fig. 107). Soon after the 

 formation of the pore, the enterocoel divides into two portions, an anterior 

 and a posterior ; the anterior remains in connexion with the water-pore 

 and constitutes the combined anterior coelom and hydrocoel, while the 



