EGHINODERMA TA— ONTOGENY 



511 



The blind end of the lengthening archenteron bends to that side, which becomes 

 the dorsal side of the larva as it rapidly grows bilaterally symmetrical (Fig. 403, A), 

 and at the same time it moves somewhat to the left side. As the hydro-enteroccsl 

 vesicle, it soon becomes entirely constricted off from the rest of the archenteron, 

 which opens outward through the blastopore (Fig. 403, B, C, D, 4, 5). 



This constriction from the archenteron, the hydro-enterocoelomic vesicle, is of 

 the greatest importance, because out of its wall arises the whole musculature of the 

 body and all the internal epithelia, i. c. the ccelomic and water vascular epithelia. 



The hydro-enterocrt'lomic vesicle increases in length, alongside of the intestine, 

 in the direction of the blastopore, and again divides into two vesicles by means of a 



Fii:. 403.— Formation of the larval mouth 

 and the hydro - enterocoelomic vesicle in the 

 gastrula larva of Synapta digitata, diagram- 

 matic (after Selenka). .1, Gastrula, the archen- 

 teron bent towards the dor.sal side ; II, archen- 

 teron, opening outward througli the hydropore ; 



C, hydro-enterocoel, constricted from the intestine; 



D, intestine, opening outward through the larval 

 mouth ventrally. 1, Segmentation cavity, bla.sto- 

 ccel ; 2, archenteron ; 3, blastopore ; i, hydropore ; 

 5, hydro-enteroccel ; 6, intestine ; 7, oesophagus ; 

 8, mouth; atit, anterior; post, posterior; v, ven- 

 tral ; (I, dorsal. 



Fin. 404. — Auricularia with the left half of 

 the ectoderm removed, from the left (after 

 Ziegler's model). The organs lying in the 

 segmentation cavity (11) are seen. 1, Cut edge 

 of the ectoderm ; 2, mouth ; 3, oesophagus ; 

 4, mesenchyme cells ; 5, mid-gut or stomach 

 intestine ; 6, anus ; 7, hind-gut ; 8, left entero- 

 ccEl, still connected with the liydrocael 10, the 

 latter showing slight indications of the first 

 radial outgrowths ; 9, dorsal pore or hydro- 

 pore ; 11, blastocrel, segmentation cavity. 



transverse constriction. The anterior vesicle (that further from the blastopore) is 

 the hydrocoelomic vesicle, which at once sends off a canal to the dorsal side, which 

 opens outwards through a pore on the left of the middle line. The canal is the 

 primary stone canal, and the pore the primary madreporite. The hydroccelomie 

 vesicle is the rudiment of all the rest of the water vascular system, and in the first 

 place, of course, of the circular canal (Fig. 404). 



The first appearance of the various structures just described does not occur in the 

 same order in all species of Holothurioidea examined on this point. The hydro- 

 enteroccelomic vesicle may become connected with the exterior through a stone canal 

 before it has divided, or even (a unique condition found in Synapta digitata) before 



