42 EMBRYOLOGICAL MONOGRAPHS. 
15. Slightly older than the preceding figure. 6, stomach. 
16. Young embryo, in which the depression } indicates the point where the five tentacles are to force their 
way through. 
17. Embryo, somewhat compressed, to show the calcareous ring, @, round the base of the stone canal, b. 
18. Embryo, seen from the ventral side, compressed. a, mouth ; 4, circular ambulaecral ring ; ¢, the five primary 
ambulacral tentacles; d, the five small calcareous tentacular appendages of the circular canal; ¢, the 
stone canal. e 
19. Embryo, seen from the mouth side. a, mouth; }, depression for the five primary ambulacral tentacles of 
the circular canal; ¢c, depression for the passage of the first pair of ventral ambulacral tentacles, 
20. Embryo, seen from the ventral side, compressed. a, mouth ; 8, cireular vascular ring ; c, ambulacral tentacle. 
21. Embryo, in which the oral tentacles have forced their way through ; seen from the dorsal side, with the tenta- 
cles protruded. 
22. Embryo, about in the stage of fig. 21, seen from the side, compressed. «a, mouth ; J, circular canal ; ¢, oral 
tentacle ; d, stone canal; ¢, Polian vesicle; f, stomach ; g, intestine. 
23. Young embryo, somewhat older than the stage of fig. 21, seen in profile. 
24. Embryo, about in the stage of fig. 23, seen in profile, under compression. a, mouth ; 6, digestive sae ; 
c, anal, and d, oral tentacles ; ¢, the vesicles of the circular vascular ring at the base of the oral tentacles ; 
f, calcareous oral ring; g, the five longitudinal water-canals ; h, water-tubes leading to the base of the 
ambulacra ; 7, ambulacral tentacles of the ventral side ; %, Polian vesicle ; Z, stone canal ; m, circular oral 
vascular ring. ; 
25. Young embryo, seen from the dorsal side, with the first-formed five oral ambulacral tentacles branching, and 
with five new tentacles placed in between them. 
26. Embryo, about in the stage of fig. 25, seen from the mouth side under compression. @, the ten oral tentacles ; 
b, mouth ; c, membranous ring round the actinostome ; d, outline of the oral calcareous ring ; ¢, circular 
water-ring ; f, Polian vesicle; g, longitudinal water-canal ; 4, branch of water-canal leading to the ventral 
ambulacral suckers ; 7, ventral ambulacral suckers ; 4, transverse muscular bands ; 7, longitudinal muscu- 
lar bands; m, stone canal; 7, anus. 
27. Embryo, seen from the dorsal side, with three pairs of ventral ambulacral tentacles, and ten branching oral 
tentacles. 
