ECHINUS. 301 
number of the former increases with age until, just below 
the ambitus, there may be from eight to twelve, according 
to the size of the test. The number of secondary and very 
small tubercles increases in like manner with the size of 
the plate. As the peristome is approached some of the 
tubercles disappear by absorption. 
Of the ten plates which form the apical system of the ~ 
test, five larger ones (Pl. IL., fig. 1, bas.) immediately 
surround the periproct, and are usually in contact with 
each other by portions of their lateral edges. These 
coincide in position with the interambulacra, and are 
called basals or genitals, from the fact that each one is 
perforated near its apex by a pore (gp.) through which the 
spermatozoa or ova escape from the corresponding gonads. 
Each basal plate usually bears three primary tubercles, 
and a variable number of smaller ones; and one, the right 
anterior (mad.), is traversed by a large number of fine 
tubules through which the madreporic tube or sand canal 
of the water-vascular system is placed in communication 
with the exterior (Pl. III., fig. 24). This plate is the mad- 
reporite. The remaining five plates (rad.) are the radials. 
They are partially wedged in between the basals, and coin- 
cide in position with the ambulacra. In some few cases 
they completely separate the basals and form part of the 
boundary of the periproct. Each one bears a variable 
number of small tubercles, and is perforated by a small 
pore through which the terminal tentacle of the water- 
vascular canal is protruded (Pl. I., fig. 1). The terminal 
tentacles are pigmented, and were formerly supposed to be 
eyes; hence the term oculars, by which the radial plates 
are commonly known. The periproct is covered by 
irregular plates, the number of which increases with the 
age of the test. It has been shown that these occupy the 
position of an originally single plate, the dorso-central. 
