148 BULLETIN OF THE 
EXPLANATION OF THE PLATES. 
LETTERING. 
a. Opening into the supposed cavity of the blastosphere, blastopore ? 
ad’, First pair of adambulacral plates. 
ad?, Second pair of adambulacral plates. 
al. Anal lobe. 
alr. Anterolateral rod. 
ar. Anterior rod. 
B: Infolded epiblast. 
cav. Cavity, segmentation cavity. 
cr. Caleareous trifid spicules. 
d. Dorsal plate, upper arm-plate. 
dc. _ Dorsocentral plate. 
eb. Epiblast. 
if Feeler, leg, paired feeler. 
ga. Stomach. 
a. Intestine. 
ar. Interradial plate. 
lp. Lateral arm-plates, adambulacral, side-plates. 
ir. Lateral rod. 
Is. Lateral ‘‘Scheibe,”’ enteroccel on left side. 
lw.  Left-water tube or hydroccel. 
m. Madreporie opening. 
mi, Inner layer or envelope of the segmentation spheres. 
m2, Outer layer or envelope. 
mb. Mesoblast. 
mbec. Mesoblastic cells } ‘‘ Mesenchyme ?” 
0. Oral plate. 
0s. Ambulacral plates, ‘‘ ossicles,” ‘‘ arm bones.” 
oe. (Esophagus. 
or. Mouth. 
pig. Pigment. 
pr. _ Posterior rod. 
ps.  Pluteus spine. 
vp. Radial plate, first radial. 
rp. t Plates of disk formed after the first radials. 
rs. Radial shield. 
