SEA-URCHIN. 263 
and along with five stout “ braces” (rotulz) and five curved 
“compasses” (radii) form “ Aristotle’s lantern,” a complex 
masticating apparatus, of whose history we know little. It 
surrounds the pharynx, and is swayed about and otherwise 
moved by muscles, many of which are attached to five 
beams which project inward from the margin of the shell 
and form a “girdle” round about the mouth. 
Fic. 137.—Dissection of sea-urchin. 
M. at the lower pole is the mouth; . at the upper pole is the madreporic 
plate; 7.7., one of the large tentacular tube-feet around the mouth; 
S.G., a skin-gill; SZ., a standard or perignath; AZ., an alveolus; 
R.V., a radial vessel, with ampulla (4.); intestine (/#z.) fixed by 
jassenterlesy P., a pedicellaria; G., a gonad: SP., spines; 7.F, 
tube-feet. 
As in other Echinoderms, the skeleton of lime is meso- 
dermic. The shell is covered externally by a delicate 
ciliated ectoderm, beneath which, in a thin layer of. con- 
nective tissue, there is a network of nerve fibres, and some 
ganglion cells. Internally, there is another thin layér of 
connective tissue, and a ciliated epithelium lining the 
body cavity. The skeleton grows by the formation of 
