ECHINODERMAT A— ONTOGENY 



515 



three apertures, that of the oral vestibule, the mouth itself lying in its floor, and the 

 anus, lie almost in the axis of the barrel-shaped body. 



In the older Auricularia stages and during the transformation into the barrel- 

 shaped larva important internal processes take place. 



Calcareous corpuscles appear earlj' (even in the younger Auricularia) in the 

 mesenchyme. In the liest known Auricularia, that of Synaxtta digitaia, these 

 bodies appear in the form of wheels in the two 

 posterior tips of the larva {cf. Fig. 404, p. 511, 

 and following). 



The hydrocoelomic vesicle assumes the form 

 of a horse-shoe with the curve towards the dorsal 

 side. On the convex side of this horse-shoe- 

 shaped vesicle five outgrowtlis ajipear. The two 

 arms of the horse-shoe then close round the fore- 

 gut, growing towards each other round it until, 

 finally, they meet and fuse (probably in the 

 right half of the body). The liorse-shoe-shaped 

 hydroccel is now the closed water vascular ring 

 surrounding the fore-gut. It continues, as before, 

 to communicate with the exterior through the 

 primary stone canal and the dorsal water pore. 



The five outgrowths of this hydroccel ring 

 now become more distinct. They are originally 

 directed forwards, but very soon, with furtlier 

 growth, bend backward, and, as the rudiments 

 of the radial canals of the water vascular . Fi'^-«3.-Diagram illustrating the 

 . ,, , , 1 ^1 1 1 11 "SB of the five ciliated rmgs of the 



system, grow further back under the body wall, Holothurian pupa froiu tlie pieces of tlie 



ciliated bands 1-7 and l'-7' of the Auri- 



iu the five radii. The rudiments of the tentacle 

 canals appear very early on the rudiments of 

 the radial canals as orally directed lateral out- 

 growths. 



The above account of the first processes of 

 differentiation in the hydrocud vesicle are those 

 found in Cucumaria Planci, the ontogeny of ^lirection in which the pieces of the ring 



of the Auricularia nnite to form the five 



cularia (after Ludwig). The pieces of 

 the ciliated band are marked by broad 

 black lines, the interruptions being left 

 clear. 8, The preoral ; 9, the postoral 

 intermediate piece of the ciliated ring ; 

 OS, mouth. The dotted lines give the 



complete rings (I-V). 



which has recently been carefully investigated. 



In other Holothurioidea, at least in Synapta 



digitata, according to former authors, the ontogenetic processes differed essentially 



from these. The first five outgrowtlis of tlie hj'droccel in Synajjta develop exclusively 



into the tentacle canals, and only after the appearance of these and alternately 



with them, five other outgrowths form the rudiments of the radial canals. 



This and certain other discoveries led to the conclusion that the radial canals 

 in the Holothurians arise interradially and only shift into the radii secondarily, 

 hence it was inferred that the tentacle canals of the Holothurioidea were homologous 

 with the radial canals of other Echinoderms, and that the radial canals of the Holo- 

 thurioidea are not represented in other classes. The above discoveries in the larva 

 of Cucumaria Planci dispose of tliis suggestion, which must always liave appeared 

 improbable to comparative anatomists. 



It is a very noteworthy fact that, in Synapta, the radial canals appear onto- 

 genetically, whereas they are wanting in the adult. 



The Polian vesicle also arises as an outgrowth of the circular canal ; in Cucu- 

 maria Planci, it forms at the point where it lies in the adult, i.e. in the left dorsal 

 interradius. 



The tube- feet arise as outgrowths of tiie radial canals, which push the ectoderm 



