10 EMBRYOLOGICAL MONOGRAPHS. 



•21. A larva at the time wlic-ii tlie skclfton begins to form ; 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 mucli older embryo, with a comparatively longer stem. 2, hood of the oral anterior 



chamber. 



26. Ti-ansverse section of a larva perhaps slightly older, anterior plane. 



27-35. Comatula rosacea. From C. "VVyville 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 lai-va 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 ; c, rigid 

 non-extensile tentacles in immediate connection with the vascular ring, ten in number, and laid up in a 

 state of complete expansion in pairs against the inner surface of the oral valves, /; d, the first pair of exten- 

 sile radial tentacles ; c, 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 ; b, series of basal 



plates ; c, first radial i>lates ; d, second radial joint ; c, third radial ; /, first brachial joint ; rj, anal plate ; 

 h, stem joint ; k, cribriform plate supporting the disk of attachment ; I, gi-anular visceral mass ; m, ccecal 

 process pas.sing from the stomach towards the papilla which indicates the position subsequently occupied 

 by the anal tube ; n, oral valve and plate. 



35. Another Pentacrinoid larva in a somewhat eariier stage than the preceding, expanded, and showing the 



arrangement of the non-extensile tentacles in connection with the oral vascular ring. 



