150 BULLETIN OF THE 



Fig. 3. Youngest pluteus with well developed posterior arms. Lateral view. 



" 4. The same, older. Lateral view. 



" 6. An older pluteus seen from the ventral side. The posterior arms are 

 well developed ; the anterior lobe is not divided. 



" 6. Ventral view of a pluteus, of about the same age as the last. 



" 7. View of a somewhat older pluteus in which the two anterior rods, ar, are 

 formed from the anterior or oral lobe. Ventral view. 



" 8. Anal lobe of a pluteus about as old as fig. 6, seen from the ventral side. 

 The appendages are not figured, c/, cell nuclei 7 Filaments,/, con- 

 necting the epiblast and hypoblast. 



" 9 Lateral view of a pluteus, a little younger than the last. 



" 10. View of a pluteus from opposite (lateral) side. This stage is a little 

 younger than the last. 



" 11. Side view of a pluteus of approximately the same age as the last. 



" 12. Pluteus in which the antero-lateral arms have begun to form. Ventral 

 view. 



PLATE VL 



Fig. 1. Lateral view of a pluteus just before the formation of the antero-lateral 



rods. 

 " 2. A figure showing the relation of the antero-lateral rod, air, when first 



formed, to the posterior, pr. 

 " 8. An older pluteus seen from the dorsal side and laterally. 

 " 4. A still more mature pluteus, seen from the ventral side. 



PLATE Vn. 



Fig. 1. Adult pluteus of E. parma, showing the young echinus forming at e. 

 Dorsal view. Camera lucida, obj. B. B., eye-piece 2, Zeiss. Reduced 

 one third in photography. Calcareous rods on the right-hand appen- 

 dages not represented in the arms. 



" 2. Adult pluteus. Ventral view. Camera B. B., eyepiece 2, Zeiss. Re- 

 duced about one third in photography. Right-hand rods not repre- 

 sented. 



" 3-17. Stages in the development of the young sea-urchin. All drawn with 

 Camera D. D., eye-piece 2, Zeiss. Reduced one third. All from 

 dorsal side of the pluteus, except Fig. 16. 



" 3. Young sea-urchin with five ambulacral tubes, ict, and a single external 

 opening, ma. Formed on the left-hand water-tube, e, in Fig. 1. 



" 4. The same, older. 



" 6. Still older stage, slightly elevated from the wall of the stomach of the 

 pluteus, which it closely hugs in Figs. 3, 4. Pigment spots of den- 

 dritic shape have appeared. A spherical calcareous body is seen at pi. 



" 6. The same, older. 



" 7. Older stage with five ambulacral tubes, seen in profile, an external open- 

 ing, ma, and two trifid limestone formations, pi. 



" 8. The satoe, older. 



