148 BULLETIN OF THE 



Fig. 18. An older larva with circumoral filiated band, and prominent anterior 



lobe. The prominences on the sides are lateral arms. 

 " 19. An older larva, in which the calcareous rods, sp, are found in the body. 



Thirty-six hours old. 

 " 20. Still older larva, one side outlined, in which the length of the calcareous 



rods has greatly increased. Three days old. 

 " 21. An older pluteus from the dorsal side. 

 " 22. The same from ventral side. 

 " 23. Dorsal view of the oldest pluteus of Ophiopholis which was studied. 



Mouth, or, seen through the oral lobe. 



PLATE IL 



All figures drawn to a scale with camera lucida. Obj. B. B. Eye-piece 2, Zeiss. 

 Reduced one half in photography. 

 Fig. 1. Ovum of Echinarachnius parma in its capsule. 



" 2. Same ovum with the primitive constriction, p, which forms the first plane 

 of cleavage. The external pigmented layer of the capsule is not 

 represented. 



" 3. Ovum in 2-ceU stage. 



" 4. Ovum in which the second plane of cleavage has begun to divide each of 

 the two cells of the 2-cell stage. 



" 5. A stage somewhat older. 



" 6. Outline of the four cells of the 4-cell stage, showing the two planes of 

 cleavage at right angles to each other. 



" 7. Ovum in 4-cell stage. 



" 8. Ovum in which constrictions have begun to form new planes of cleavage, 

 which later divide the four cells of the 4-cell stage to form the 8-cell 

 stage. 



" 9. Ovum in the 8-cell stage. 



" 10. The same seen in a plane at right angles to the last. 



" 11. Ovum in 8-cell stage showing the segmentation cavity, cav. 



" 12. Segmented ovum of a stage with more cells than the 32-cell stage. Two 

 of the cells are represented with nuclei. 



" 18. The same, older, with segment spheres more angular and segmentation 

 cavity shown. 



" 14. Blastosphere, "planula stage." The cells have taken the form of a 

 hollow sphere. The larva has left the egg-capsule. 



" 15. The same showing the flattening at one pole preparatory to an invagina- 

 tion. (Optical section, in which the cells are not shown in tlie hemi- 

 sphere turned to the observer.) 



" 16. Young gastrula with partially infolded blastoderm, forming the stomach, 

 (/a. 



" 17. Older gastrula. Outlines of blastodermic cells not represented. 



