8 Echinoderma. 



the vestibular sac is not pushed forward towards the superior extremity, but 

 retains its position on the ventral face and also its communication with the ex- 

 terior ; while the calyx is not rotated so as to become terminal, but remains attached 

 to the stem by its side, eventually, however, gaining its normal position. In other 

 cases again the ciliated larva, instead of becoming fixed, developes tentacles on 

 its ventral face without any vestibular invagination and creeps about by means of 

 them as described by Busch. This may be called the Cucumaria-mo&e of deve- 

 lopment, while the normal larvae have a vestibular cavity, like the young Synapta. 

 Bury describes the early stages in the development of Antedon rosacea. Seg- 

 mentation commences some 3 hours after fertilisation and proceeds regularly, the 

 blastosphere being completed in 12 hours and invaginated 5 hours later. The hy- 

 poblast cells are at first smaller than those of the epiblast and give rise to the 

 mesoblast. The blastopore is largest about the middle of the second day, and 

 narrows to an elongated slit which gradually closes from one end to the other. 

 The closure is complete 40 hours after fertilisation and soon afterwards the blasto- 

 pore becomes uniformly ciliated, while the archenteron is divided transversely in- 

 to an anterior vesicle, the mesenteron, and a posterior one, the enterocoel pouch, 

 the former of which is slightly the larger. By the fourth day the 5 ciliated bands 

 appear, together with a terminal (anterior) tuft of cilia. Two depressions are 

 formed on the thickened ventral surface, a circular anterior one, the pre-oral pit, 

 and an elongated posterior one, the larval mouth. The enterocoel pouch is 

 constricted to form the right and left coelomic pouches, extending from left to 

 right, and the mesenteron curves round the constricted part towards the posterior 

 end; while it also puts out 2 new pouches to form the hydrocoel (left and 

 ventral), and a median anterior body- cavity (water-tube, Barrois) , which 

 runs forwards into the pre-oral lobe. Posteriorly it remains in connection with 

 the hydrocoel after they have both been cut off from the mesenteron, the horns 

 of which meet and fuse behind the constriction separating the right and left coe- 

 lomic pouches, so as to form a complete ring. The 2 pouches communicate for 

 some time by a hollow cord which perforates this ring ; but it is soon resorbed, 

 and the mesenteron takes the form of a disc flattened between the 2 coelomic 

 pouches. In the free-swimming larva the ciliated bands, with the region of the 

 terminal anterior tuft, and both the ventral depressions, which are also ciliated, 

 are all white, the remainder of the embryo being yellowish brown. The anterior 

 band is usually incomplete ventrally, not passing round the pre-oral pit ; and on 

 the dorsal side it is very close to the terminal tuft of longer cilia, which is di- 

 rected forwards in swimming. The 2 middle bands are deflected round the ends 

 of the larval mouth, the 3 d one fusing posteriorly with its ciliated edge. The 

 opening of the water-pore generally appears on the 7 th day as a white spot 

 between the 3 d and 4 th ciliated bands. The ciliated cells have striated outer 

 borders and small deeply staining nuclei towards their inner ends; while the re- 

 maining ectoderm cells have large median nuclei, staining but little. Beneath the 

 ectoderm cells round the edge of the pre-oral pit, down the sides of the larval 

 mouth and beneath the whole of the anterior pole, is a layer of fine fibrils which 

 are probably nervous. - The left coelomic pouch now assumes a posterior and 

 ventral position, while the right one becomes more anterior and dorsal, and the 

 mesentery between them changes from a longitudinal to an oblique position. The 

 hydrocoel now takes a horse-shoe form, open towards the left, and lying between 

 the mesenteron and the ventral ectoderm. The ring becomes nearly complete and 

 5 tentacles appear on its ventral surface. The anterior body-cavit}' separates 

 from it and opens to the exterior by the water-pore, which, together with a small 

 portion of the pore-canal leading from it, is lined by ciliated cubical cells. The 



