VIII 



ECHINODERMA TAONTOGEN Y 



533 



the hydroccel vesicle (Fig. 442). This at once becomes entirely distinct, and 

 lengthens out anteriorly below the left anterior euteroccel vesicle. At its outer left 

 edge it then produces five outgrowths, the rudiments of the radial portions of the 

 water vascular system. Between the fourth and fifth outgrowths (reckoning from 

 before backward) a dorsally directed diverticulum further grows out from the 

 hydroccel vesicle, which, after 



a very short course, comes in ,=- \ ,/,// 



contact with the left anterior 

 enteroccel vesicle, and opens 

 into it immediately below the 

 aperture of the water pore. 

 This diverticulum is the rudi- 

 ment of the stone canal. Its 

 connection with the dorsal pore 

 (madreporite) is thus secondary, 

 and is brought about by means 

 of the left anterior enteroccel, 

 which no doubt becomes the 

 ampulla. 



The long hydroccel vesicle, 

 with its five outgrowths, then 

 clasps the larval oesophagus 

 like a halter, and grows round 

 it ; this larval oesophagus 

 apparently becomes the defini- 

 tive oesophagus, Avhile no 

 definitive anus replaces that 

 of the larva. 



First appearance of the 

 plates of the skeleton. Soon 

 after the formation of the stone 

 canal, ten skeletal plates appear 

 on the Pluteus larva, five on 

 the left and five on the right 

 side, i.e. above the left and 



right posterior ccelomic vesicles. The five on the right side are the radials of the 

 apical system ; the five on the left are the terminals. In the middle of the right 

 side the rudiment of the central plate then appears, and on the left side, immediately 

 in front of the water pore, another plate appears, which is the fifth oral, the one 

 which becomes the madreporitic plate. The madreporite thus belongs ontogenetically 

 to the oral system of plates. The other parts of the skeleton form only after the 

 metamorphosis. 



post 



FIG. 442. Dorsal aspect of a young Ophiuroid Pluteus, 

 to illustrate the hydro -enteroccel (after Bury). 1, Larval 

 oesophagus ; 2, left anterior enterocoel ; 3, hydropore ; 

 4, hydroccel ; 5, left posterior enterocoel vesicle ; 6, stomach- 

 intestine ; 7, right posterior enteroccel vesicle ; 8, right 

 anterior enterocoel vesicle. 



F. Ontogeny of the Crinoidea. 

 The Ontogeny of Antedon alone has been investigated. 



1. Embryonic Development. 



Here also a coelogastrula is formed by the invagination of a cceloblastula. The 

 transverse slit-like blastopore indicates the posterior end of the larva, which at a 

 later stage becomes bilaterally symmetrical. The segmentation cavity is filled by 

 an albuminiferous gelatinous mass (gelatinous nucleus). 



