VIII 



ECHINODERMA TA 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 early (even in the younger Auricularia) in the 

 mesenchyme. In the best known Auricularia, that of Synapta digitata, these 

 bodies appear in the form of wheels in the two 

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

 and following). 



The hydroccelomic 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 outgrowths appear. 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 horse-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 further 

 growth, bend backward, and, as the rudiments 

 of the radial canals of the water vascular 

 system, grow further back under the body wall, 

 in the five radii. The rudiments of the tentacle 

 canals appear very early on the rudiments of cularia (after Ludwig). The pieces of 



the radial canals as orally directed lateral out- the ciliated baml are lliark ed by broad 



. , black lines, the interruptions being left 



'" 1S> clear. 8, The preoral ; 9, the postoral 



The above account of the first processes of intermediate piece of the ciliated ring ; 



differentiation in the hydrocoel vesicle are those os, mouth. The dotted lines give the 



found in Cucumaria Planci, the ontogeny of direction in which the pieces of the ring 



i -, i ,1 i c TI , ", j of the Auricularia unite to form the five 



which has recently been carefully investigated. 



J J complete rings (I-V). 



In other Holothunoidea, at least in Synapta 



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

 from these. The first five outgrowths of the hydrocoel in Synapta 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 this suggestion, which must always have 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 the radial canals, which push the ectoderm 



FIG. 413. Diagram illustrating the 

 rise of the five ciliated rings of the 

 Holothurian pupa from the pieces of the 

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



