STRUCTURE AND FUNCTION OF THE SPHAIRIDIA. 51 
ing fluid, which now adheres as a dry film to the sides of the canals. The 
calcareous substance is not in the least coloured. 
Fie. 11.—From the same preparation as Fig, 10. The canals are arranged 
fan shape (in optical section), and are to be traced from the reticulate canal 
system of the neck to the surface of the globule, on which most of them 
open. 
Fig. 12.—Cells from a spheridium of Echinus melo. a. A large cell, 
with chromatophores and vacuole from beneath the epithelium of the base of 
the organ. 4. Surface view of five epithelial cells. From an osmo-acetic- 
carmine preparation. 
Fic. 13.—An optical section of a portion of the epithelial cover of a 
spheridium of Echinus esculentus. At xxx are shown the ends of nerve- 
cells, which pass out to the surface of the organ through the canals in the 
calcareous substance (here dissolved away) and between the epithelial cells. 
They end at the surface in knot-like enlargements. Chromic-osmic-acetic 
solution, stain Ranv. pic. car. 
Fie. 14.—An optical section and a surface view of the epithelium of 
Echinus melo. a. Optical section from the apex of the organ, showing 
the numerous thin layers of slime or mucus surrounding the globule in the 
living state, and through which the cilia pass. 4. Surface view: in three of 
the cells are shown the chromatophores, which are of common occurrence. 
Fie. 15.—A fresh specimen of a spheridium of Strongylocentrotus 
droebachiensis. The upper end of the calcareous globule is broken off, 
and the epithelial layer is ruptured. The nervous system is seen to extend 
into and fill part of the canals. 
Fic. 16.—A spheridium from Spatangus purpureus, treated with 
caustic soda. Surface view, showing the openings of the canal system. 
Fic. 17.—The same as Fig. 16, seen in optical section. The figure shows 
three of the principal canals, the canalicular network of the neck (neither of 
which open on the surface in this globule), and also the lateral branches of 
the left central canal; the side cells establish communication with the 
epithelial layer. 
_ Fie. 17,a.—«. A portion of the left stem shown in Fig. 17, and one of its 
side branches, with its outer opening. z. A portion of the canalicular network 
of the neck further magnified. The shaded parts represent the canals. 
Fic. 18.—A living spheridium of Spatangus purpureus, treated with 
dilute acetic acid until the superficial layers of the calcareous body are partly 
dissolved away. The epithelial cover is ruptured to expose the globule. A 
part of the canals are left as grooves in the new surface, appearing as notches 
at the margin of the globule. 
Fig. 19.—An optical section of a spheridium of Echinus melo, showing 
the structure of the calcareous part. This globule is composed of wavy con- 
centric lamelle and radiating cones. 
