VIII 



EGHINODERMA TAONTOGEN Y 



53* 



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

 lengthens out anteriorly below the left anterior enteroocel 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 

 hydrocoel 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 hydrocoal 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, while no 

 definitive anus replaces that 

 of the larva. 



First appearance of the 

 plates of the skeleton. Soon 

 after the formation of the stone 



post 



FIG. 44-2. ^Dorsal aspect of a young Ophiuroid Pluteus, 

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

 (esophagus ; 2, left anterior enterocael ; 3, hydropore ; 

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

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



canal, ten skeletal plates appear 



on the Plvtevs larva, five on 



the left and five on the right anterior enterocoel vesicle. 



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 



iu 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. 



F. Ontogeny of the Crinoidea. 

 The Ontogeny of Antedon alone has been investigated. 



1. Embryonic Development. 



Here also a ccelogastrula 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). 



