VI. Holothurioidea. 25 



ciliated cells of the nerve bands are less closely packed than in the ciliated bands, 

 being generally in 2 or sometimes 3 rows ; and beneath their lower ends, which 

 are ill defined, are nerve fibrils but no ganglion cells, as described by Metschni- 

 koff. The musculature of the fore -gut is developed from the mesenchym cells 

 around it. These send out extensions which unite and form ring fibres, so that 

 one cell takes part in the development of many fibres. A longitudinal muscula- 

 ture, hitherto unnoticed, also appears within the ring fibres of the fore-gut, the 

 movements of which are almost rhythmical and seem to serve respiratory as well 

 as nutritive functions. The mesenchym cells also give rise to the calcareous 

 armature, the first traces of which appear in the left lower lobe, the spheres 

 before the wheels. Traces of the plates of the oral ring early appear by the hy- 

 drocoel-rosette, as minute rods which alternate with the primary tentacles. 

 During the change to the pupa stage the hydrocoel approaches the middle line 

 of the fore-gut and becomes more horizontal, moving round the point of insertion 

 of the stone-canal. The cells of the ciliated bands lose their continuity at 7 pairs 

 of points, though the grooved support of mesenchym cells still remains, and the 

 bands are transformed into 5 ciliated rings. Four isolated pieces of them sink 

 into the deep oral funnel carrying with them the nerve bands, while the cells of 

 the adoral ciliated band spread out as an epithelium on the wall of the fore-gut. 

 The oral funnel narrows to a mere cleft which moves from the ventral side to the 

 front end, and the larva shrinks in all dimensions, often to less than half its 

 former size, chiefly at the cost of the mesenchym. All the ciliated rings close on 

 the ventral side, the anus shifting its position a little, so as to appear behind the 

 fifth, while the front one is the last to close. - - As the oral funnel narrows, the 

 isolated portions of the ciliated bands which have sunk into it, are brought close 

 together and form a compact oral disc. Beneath this are the nerve bands 

 which sink deeper to the point where the oral funnel passes into the true mouth, 

 and unite round the latter to form the oral nerve ring. Branches from this ascend 

 the inner side of the tentacles and terminate in the substance of the oral disc 

 which overlies the hydrocoel-rosette and gives rise both to the sensory epithelium 

 of the tentacles and the 10 otocysts and to the atrial epithelium. The ambula- 

 cral nerves appear later than the tentacle-nerves and lie on the outer side of the 

 water- vessels. The nerve ring, like the larval nerve bands producing it, consists 

 of protecting cells outside and fibres within. It is at first ectodermal, but is gra- 

 dually enclosed by a cellular layer which is derived from the oral disc and comes 

 to be separated from the nerve ring by mesenchym cells. The closure of the 

 hydrocoel -ring occurs on the former ventral side of the larva; the 5 water- 

 vessels alternate with the tentacles and arch upwards over the pieces of the oral 

 ring, between which 5 new ones appear, while the Polian vesicle is developed. 

 The whole water-system is lined by a low cylindrical epithelium, and ciliary 

 movement is visible about the middle of the pupa stage, in the tentacles first. The 

 musculature of the water-vessels is derived from the hydrocoel-epithelium, the 

 longitudinal fibres of the tentacles appearing first, and then the circular fibres of 

 the Polian vesicle and water-ring, with isolated longitudinal fibres in the radial 

 vessels. - The 2 coelomic pouches growing round the gut meet and unite 

 ventrally, owing to the fusion of their walls, but on the dorsal side these persist 

 and form the mesentery which remains as an indication of a primitive bilateral 

 type and supports the stone-canal. A thin sheet of mesenchym cells separates 

 the epithelium of the gut from the visceral layer of the coelomic epithelium, but 

 a thicker sheet separates the parietal layer from the body wall. At the base of 

 this peritoneal epithelium in both layers is developed the musculature, first the 

 circular and then the longitudinal fibres. The latter are continuous beneath the 



