ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 961 



trace of the ventral dimple (fossette) of the larva. Indications of the 

 ciliary rings also appear. The superior vesicle divides into two 'peri- 

 toneal sacs, while the inferior vesicle is continued upwards with the gut, 

 still quite closed. Thereafter the inferior vesicle divides into two parts, 

 the water-vascular vesicle on the left, the stone-canal on the right. Com- 

 pared with other Echinoderms, the peduncular end is anterior, the 

 calycine or blastopore end is really posterior. The gut separates from 

 the water-vascular vesicle, the latter becomes situated on the top of the 

 visceral mass with the stone-canal on the side. The visceral mass then 

 acquires the typical form seen in other Echinoderm larvee, with the gut 

 surrounded by the two peritoneal sacs, and surmounted by the water- 

 vascular vesicle. It alters its position from being parallel with the main 

 axis to being oblique, and eventually becomes parallel with the lesser 

 axis. The accurate details of the displacement can hardly be compressed. 

 With the appearance of the skeleton various changes have to be noticed. 

 The horseshoe-shaped water-vascular vessel is divided into five lobes; 

 the stone-canal is separated from the latter, and comes to lie transversely 

 below the water-vascular horseshoe. The two peritoneal sacs become 

 unequal and lie obliquely to one another. In the middle of the compact 

 mass of mesenchyme the cells begin to be stratified in horizontal strands. 

 At the same time appear the peduncular cord and the fixing depression. 



The free larva of the seventh day has a solid stalk cord, formed from 

 concentrically disposed cells, and originating from mesenchyme. The 

 fixing dimple (fossette), situated just at the side of the terminal cap, 

 forms, like the latter, an extremity of the larva, viz. the extremity of the 

 calcareous axis of the stalk, while the cap forms the true extremity of 

 the larva. There is here a coincidence of the two longitudinal axes of 

 the Pentacrine and of the larva. In the free larva, the cap has given 

 origin on its margins to a fifth circle, the visceral mass is slightly 

 turned towards the superior pole, and the hollow (echancrure) of the 

 water-vascular horseshoe, at first directed upwards, is turned downwards. 



(2) Metamorphosis. — The ectoderm loses its divisions and becomes a 

 rounded delicate sac ; the ventral dimple is transformed into a thick 

 mass, which invaginates to give origin to the tentacular chamber ; the 

 visceral mass continues to become more oblique until it has changed its 

 transverse position for one in which it is directed towards the superior 

 pole of the embryo. The intestine expands into a spherical mass full of 

 nutritive vitellus. The stone-canal opens by a narrow duct to the left. 

 The two peritoneal sacs are decidedly superposed ; the original left 

 becomes the peri-^cesophageal horseshoe, the original right forms the 

 peritoneal cap. The ventral curvature becomes accentuated, and the 

 stratified portion of the solid mass of the stalk extends from the centre 

 through the entire mass. 



(3) Post- embryonic Development. — The young Pentacrines at all 

 stages appear in profile to be incurved on one side. This is really a 

 continuation of the ventral curvature of the embryo. Stalk and calyx 

 are gradually defined by a thinning off of the entire posterior region. 

 After becoming a simple sac, the ectoderm passes through four stages : — 

 (a) the cells are small and closely apposed ; (h) they are slightly 

 elongated and separated ; (c) they are reduced to delicate fibres, with 

 the slight residue of the cell-body proper at their internal extremity, 

 and surrounded by the preponderant structureless substance filling up 

 the interspaces ; (d) the cells become like true connective cells, while 



