10 EMBRYOLOGICAL MONOGRAPHS. 
21. Alarva at the time when the skeleton begins to fori; the diagonally transverse dark stripe running across the 
digestive cavity indicates the position of the mesentery. 
22, 23. Sagittal section of a young embryo with a stem. 
24. Frontal section of a similar larva ; the two layers of the rudimentary ring canal and tentacles begin to separate. 
25. Median section of a much older embryo, with a comparatively longer stem. 2, hood of the oral anterior 
chamber. 
26. Transverse section of a larva perhaps slightly older, anterior plane. 
27-35. Comatula rosacea. From C. Wyville Thomson. 
27. Dorsal view of a pseudembryo about in the stage of fig. 9. 
28. Lateral view of a pseudembryo somewhat older than the preceding figure ; the ciliated bands are disappearing. 
29. The pseudembryo is losing its special organs of assimilation and locomotion, and is passing into the Pentacri- 
noid stage. 
30. The youngest Pentacrinoid stage. 
31. Pentacrinoid larva immediately after the complete separation of the oral valves, expanded. 
32. A portion of the oral disk of the same stage seen from above, in a state of complete expansion ; a, patent oral 
aperture bounded by a ring of contractile tissue, and showing yellow, richly ciliated granular folds, arranged 
somewhat spirally on the walls of the digestive cavity ; b, central ring of the radial vascular system ; ¢, rigid 
non-extensile tentacles in immediate connection with the vascular ring, ten in number, and laid up ina 
state of complete expansion in pairs against the inner surface of the oral valves, f; d, the first pair of exten- 
sile radial tentacles ; ¢, azygous radial extensile tentacle leading out of the growing arm to its bifurcation, 
and giving off pairs of tentacles of the same series from its base. 
33. Pentacrinoid in the same stage as fig. 31, the cup closed. 
34. Pentacrinoid larva immediately before the expansion of the ventral disk ; a, centrodorsal plate; , series of basal 
plates ; ¢, first radial plates; d, seeond radial joint ; ¢, third radial ; f, first brachial joint ; g, anal plate ; 
h, stem joint ; &, cribriform plate supporting the disk of attachment ; 7, granular visceral mass ; m, ececal 
process passing from the stomach towards the papilla which indicates the position subsequently occupied 
by the anal tube ; 2, oral valve and plate. 
35. Another Pentacrinoid larva in a somewhat earlier stage than the preceding, expanded, and showing the 
arrangement of the non-extensile tentacles in connection with the oral vascular ring. 
