ECHINUS. 321 
test of the young Echinus. The apical half of the test is 
said to be formed directly from the dorsal ectoderm of the 
pluteus. The four pairs of arms gradually disappear, and 
their skeletal rods are absorbed. 
The young Echinus takes over the larval stomach and 
the enteroccelic and hydroccelic vesicles, but the larval 
mouth and anus are succeeded by corresponding structures 
The water 
pore, by which the original hydroccelic vesicle opened to 
Pedicellariz 
formed independently in the young urchin. 
the exterior, becomes the madreporite. 
appear early, even before the close of the pluteus stage. 
Later on certain of the plates of the test and parts of the 
masticatory apparatus appear as minute lattice-lke plates. 
Advanced plutei of Hchinus esculentus have been taken in 
the tow-net, in Port Erin Bay, early in June, and the 
young Echini probably take to their adult mode of life 
towards the end of that month or the beginning of July. 
EXPLANATION OF THE PLATES. 
Reference Letters. 
ad.m.t. adductor muscles. 
am.t.f. ampulle. 
an. anus. 
ap. aperture of gland. 
arch. archenteron. 
au. auricula. 
ax.sin. axial sinus. 
bas. basal plate. 
blas. blastopore. 
cen.pl. central plexus. 
c.ep.c. epineural canal. 
c.p.h.c. circ. pseudhzemal canal. 
c.t.f. canals of tube feet. 
c.t.l, connective tissue. 
cu. cuticle. 
c.w.v. circular water vessel. 
d.v. dorsal vessel. 
e.b.c. epithelium of body cavity. 
ect. ectoderm. 
ep. epithelium of intestine. 
ex.g. external gills. 
gl.c. gland cells. 
gl.s. glandular sac. 
gon. gonad. 
gp. genital pore. 
hy.ent. hydro-enteroccel. 
wmf.s. inferior spiral of intestine. 
mt.alv.m. inter-alveolar mus. 
