VIII 



ECHINODERMA TA— ONTOGENY 



543 



Alimentary canal. — Tlie mouth from the very first does not lie exactly at the 

 centre of the oral disc, but somewhat eccentrically in the interradial area bordered 

 by the first and fifth radii. 



The stomach becomes a capacious sac, and the yolk-like mass contained in it is 

 gradually absorbed. The hind-gut arises out of it (in interradius III-V), being 

 broad at the base, and then thinning into a 

 tube -which has the following course. Viewing 

 the larva from the oral pole, the hind-gut runs 

 (in the direction of the hands of the clock) in 

 the horizontal mesentery, near the body wall, 

 through the interradial space IV- V, then runs 

 across radius V, and immediately after opens 

 outwards in the interradial space V-I, through 

 the anus which has in the meantime broken 

 through the calyx laterally. This is the same 

 interradius in which the hydropore lies, on 

 the original ventral side of the bilaterally 

 symmetrical larva. The ectoderm takes no 

 part in the formation of the anus. 



In the coelom sacs profound changes are 

 going on, which may be briefly summarised 

 as follows. 



{a) The chambered sinus gives up all 

 connection with the original right, now the 

 aboral ccelom sac. 



{h) The mesenteries (both the principal 

 and the longitudinal accessory mesentery) are 

 completely resorbed, and as a consequence 

 the right and left cceloms unite to form one 

 large body cavity. 



(c) The trabeculse (of endothelial origin) 

 become very strongly developed, and traverse 

 the body cavity in all directions as a network. 



{(l) The axial organ becomes differentiated 

 as an independent solid cell strand, lengthens 

 till it reaches the tegmen calycis (oral disc), 

 and at a later stage becomes hollow. 



(c) In the parietal sinus, which comes to 

 lie quite in the body cavity, two sections 

 become more and more distinct ; the one 

 vesicular, and the other a narrow canal-like 

 section opening outwards through the hydro- 

 l)ore. The former, into which the primary 

 stone canal enters, loses its independent 

 endothelium, and the thin wall which 

 separates it from the ccelom also probably 

 disappears, so that it ceases to exist as a 

 separate cavity. The stone canal now opens 



into the general body cavity, which is thus in communication with the exterior 

 through the narrower section of the original parietal sinus, and through the 

 hydropore in the anal interradius. 



In the hydroccel, the water vascular ring completely closes. The whole of the 

 musculature of the hydroccel is formed by the hydroccelomic epithelium itself. The 



Frci. 454.— Calyx of a decalcified larva 

 of Antedon, five weeks old, with extended 

 tentacles, from the left and lower side (after 

 Seeliger). I-V, The five radially placed 

 primary saccnli ; 1, axial organ ; 2, right 

 (aboral) ccelom ; 3, principal mesentery, 

 between the right (aboral) and the left (oral) 

 coelom ; 4, hind - gut which follows the 

 stomach ; 5, oral cwlom ; 6, hydroccel ring ; 

 7, two of the ten secondary tentacles ; 

 S, tentacle papilte ; 0, primary tentacles 

 (only seven of the total number, fifteen, are 

 represented) ; 10, oral lobes ; 11, stone canal ; 

 12, hydropore ; 13, oesophagus ; 14, stomach ; 

 15, continuation of the chambered organ in 

 the stalk (16). 



