MUSEUM OF COMPARATIVE ZOOLOGY. 151 



Fig. 9. The same, still older. 



" 10. An older stage, with numerous limestone bodies of dendritic shapes. 



" 11, An older condition of the sea-urchin, in which the ambulacral tubes 

 have developed very considerably, and the dendritic calcareous body 

 is enclosed in a transparent " ceU," pi, resembling " Hohlkehlen," 

 described by Johannes. Mliller. 



" 12, 13. Still older stages, similar to Fig. 11. 



" 14. A young stage, in which the pluteus is so twisted that a central body, 

 cp^, "centrale," and five peripheral bodies, pi, are shown. At am 

 are the ambulacral tubes. The view is at right angles to that of 

 Figs. 3-13. ma is turned out of sight. The bodies, pi, may be five 

 radial water-tubes. 



" 15. A sea-urchin of about the same age as Fig. 13, showing its relation to the 

 anal rods, rd, of the pluteus. 



" IG. View of a young sea-urchin from the ventral? side, submitted to slight 

 pressure. At cpl there is a central plate without calcareous deposits. 

 Around this plate is a ring of five polyhedral plates, in which, pt, there 

 is a deeper stellate calcareous system, " stellate cells," and superfi- 

 cial, " lace-work cells or rods." If this is a ventral view, and we are 

 looking at the plates from below, the stellate rods would be superfi- 

 cially placed on the test, and the " lace-work " rods more profound. 

 The lace-work of rods would then be the beginning of the plates of 

 the test of the sea-urchin. 



" 17. A sea-urchin older than the last, with attachment to its pluteus. 



" 18. The mouth and adjacent region of the anterior rods of a pluteus of aoout 

 the same age as Fig. 2, showing the muscular fibres at the end of 

 the dotted line without letters. The antero-internal rods are moved 

 in part by these muscles. Free-hand drawing. 



PLATE VIII. 



Fig. 1. View of the surface of the pluteus between the posterior rods, pr, and the 

 anterior rod, showing a structure similar to the so-called loop, t, 

 described by A. Agassiz in Strongylocentrotus. 



" 2. The same " loop ' with the external opening partially closed, and the 

 whole structure more retort-shaped. 



" 3. Relation of the " loop "-like structure to the posterior rod, pr. 



" 4. The relation of the same structure, " loop," to the water-tube, wt. 



" 5. A view of the same very much reduced in size, with the orifice almost 

 closed. 



" 6. The arrangement of the calcareous rods in the body of the pluteus. Soft 

 parts removed. Lateral view. Anal pole at left of the sketch. 



" 7. The relation of tlie infolded part of the external surface to the water- 

 tube. 



" 8. A well formed sea-urchin with spines. Dorsal view of pluteus. Arms 

 of pluteus removed. 



" 9. An older sea-urcliin. Ventral view. Arms of pluteus removed. 



