52 HOWARD AYERS. 
Fic. 20.—A spheridium of Echinus esculentus, to illustrate the dis- 
tribution of the cilia, chromatophores, and the relation of the parts of the 
organ. a. The basal articulation (a knob of the test). 4. The joint, composed 
of muscle-cells aud fibrous bands. /. The limiting line between the epithelium 
and tissues of the joint. c. The neck. g. The red and brown chromato- 
phores most plentiful in the region of neck and base. d. The head of the 
spheridium. e. The epithelial covering. The thickened ring of epithelial 
cells surrounding the neck and the base. g’. A large cell, such as is found 
frequently in the base. 4. The green chromatophores (chlorophyll?). 2. 
Finger-shaped processes of the base. The base also frequently bears one or 
two small pedicellaria. ww. Patches of vibratile cilia. 
Fic. 21.—Muscle-cells from the joint of a spheridium of Strongylocen- 
trotus lividus. From an organ macerated in osmo-acetic solution (Hert- 
wig’s), stained in carmine mounted in glycerine. 
Fic. 22.—Hpithelial ceils from Echinus melo. Chromic-acetic maceration 
preparation. Ranvier’s carmine, glycerine. 
Fic. 23.—The central part of a transverse section of a spheridium of 
Echinus esculentus, through the neck, showing the remains of the cal- 
careous plates (a), and the nervous matter of the canals (4). 
Fic. 24.—A surface view of the epithelial layer of a spheridium of 
Kchinus melo, showing chromatophores. 
Fic. 25.—Muscular and connective-tissue cells (a, 4, 6'), and nerve- ,-cells (ce), 
from a spheridium of Echinus melo, treated with chromo-osmo-acetic 
(Flemmings’s) solution ; stained in carmine, mounted in glycerine. 
Fie. 26.—A transverse section of the spheridium of Echinus melo, 
through the head of the organ, after treatment with chromo-acetic, carmine, 
clove oil, paraffine. 
Fic. 27.—Three stages in the death of the cilia of Spherechinus brevi- 
spinosus. a. The slowly-vibrating cilia immediately after the action of the 
chromo-osmo-acetic solution. 4. The resting cilia just after the cessation of 
motion. c. Five minutes after death. A little later the cilia have curled close 
up to the epithelial surface. 
7 
