Fig. 
“ 
“ec 
“ 
“ 
9. 
10. 
11. 
MUSEUM OF COMPARATIVE ZOOLOGY. 151 
The same, still older. 
An older stage, with numerous limestone bodies of dendritic shapes. 
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 “ cell,’ pl, resembling “ Hohlkehlen,” 
described by Johannes Miiller. 
12, 13. Still older stages, similar to Fig. 11. 
14. 
15. 
16. 
17. 
18. 
ou ce 
A young stage, in which the pluteus is so twisted that a central body, 
c pl, “centrale,” and five peripheral bodies, pl, 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, pl, may be five 
radial water-tubes. 
A sea-urchin of about the same age as Fig. 13, showing its relation to the 
anal rods, rd, of the pluteus. 
View of a young sea-urchin from the ventral? side, submitted to slight 
pressure. At cpi there is a central plate without calcareous deposits. 
Around this plate is a ring of five polyhedral plates, in which, pl, 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. 
A sea-urchin older than the last, with attachment to its pluteus. 
The mouth and adjacent region of the anterior rods of a pluteus of about 
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 VIIL 
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. 
The same “loop’ with the external opening partially closed, and the 
whole structure more retort-shaped. 
Relation of the “loop ”-like structure to the posterior rod, pr. 
The relation of the same structure, “loop,” to the water-tube, wt. 
A view of the same very much reduced in size, with the orifice almost 
closed. 
The arrangement of the calcareous rods in the body of the pluteus. Soft 
parts removed. Lateral view. Anal pole at left of the sketch. 
The relation of the infolded part of the external surface to the water- 
tube. 
A well formed sea-urchin with spines. Dorsal view of pluteus. Arms 
of pluteus removed. 
An older sea-urchin. Ventral view. Arms of pluteus removed. 
