VIII 



ECHINODEBMA T A— ONTOGENY 



531 



tips of the growing arms (Fig. 441, t^ - Q. Five others appear within the anteriorly 

 open curve made by the five terminals, and alternate with these latter ; these are 

 the primary interradials (basals) of the dorsal surface of the Asteroid disc (fia-^ - ha^). 

 One of these (6a,) always lies on the right near the dorsal pore, and, growing round 

 this pore later, becomes the madreporitic plate. The eleventh plate lies in the 

 centre of the two curves just mentioned, and is the rudiment of the central plate (cc). 



The basals and the central appear on the right side of the larva over the right 

 enterocoel. The relation of the terminals to the enterocojl has not yet been 

 certainly ascertained. It has 

 been proved that in the Bipiii- 

 iiaiia they appear even before 

 the rudiment of the five hydro- 

 coel outgrowths, above the left 

 enterocoel. 



Metamorphosis of the larva 

 into the young Asteroid. — 

 This is throughout a continuous 

 process. Only two parts of the 

 larva are not taken over by 

 the young Asteroid, viz. the 

 larval organ and the larval 

 oesophagus, which are gradually 

 resorbed. The anus of the 

 Asteroid does not indeed de- 

 velop out of that of the larva, 

 but at the same point. 



The last remains of the 

 larval organ are found on the 

 ventral side of the young Fio. 441.— Asterina gibbosa, young Asteroid ten days 

 Asteroid lying excentrically old, dorsal view (after Ludwlg). I-V, The aiitiambulacral 

 in that iuterradius in which ^""^ rudiments ; I, interradius of the larval organ (lo) ; m, in- 

 the hydroocel closes to form torradii^s of the madreporite (mp) ; tej - te„ the five basals ; 



■^ (j - Eg, the five terminals ; ce, central, 



the water vascular ring ; view- 

 ing the body apically this interradius follows the madreporitic interradius on the 

 right (cf. Figs. 440, 441 ; the arrows indicate these interradii). 



The mouth and oesophagus of the young Asteroid arise by the outgrowth from 

 the left side of the archenteron, mentioned above, reaching the body wall and finally 

 breaking outward through it (thirteenth or fourteenth day). The oesophagus is then 

 gi-own round by the hydroccel, which closes to form the water vascular ring. Only 

 shortly before this takes place does the hydroocel become entirely constricted off 

 from the enterocoel, and the dorsal pore comes into direct connection with the 

 stone canal. 



The intestine widens into a sac,'five radially placed outgrowths appearing in it, 

 directed towards the rudiments of the arms. At the point where the larval anus 

 formerly lay, in the interradius between the first and second apical brachial rudi- 

 ments, the definitive anus breaks through. 



The two curves formed by the five apical (antiarabulacral) and the five oral 

 (ambulacral) arm rudiments approach one another more and more, the zone of the 

 body wall which separates them (and which is, with regard to the animal, equatorial) 

 becoming continually narrower. Finally, the edges of the apical and those of the 

 oral rudiments touch to form the young Asteroid. During this process the arm 

 rudiments unite in the following peculiar manner : Number 1 unites with II, 

 2 with III, 3 with IV, 4 with V, and 5 with I. 



