368 
TOPULAR SCIENCE REVIEW. 
Thus there are altogether twenty vertical (or longitudinal) 
series of plates, ten of these being ambulacral and ten being 
interambulacral. 
The relation of these alternating tracts to the circle of ocular 
and genital plates is such that an ocular plate is placed at the 
summit of each ambulacrum, while a genital plate stands at 
the top of each interambulacrum. 
One of the five genital plates is larger than the rest, and has 
a porous space, which gives a worm-eaten look to its surface. 
This modified genital plate is termed the maclreporic tubercle , 
and is of course, like the other genital plates, interambulacral 
in position, or, as it is sometimes called, inter-'i'adial. 
At the opposite, or oral, pole (beyond the corona formed of 
the ambulacra and interambulacra) are the buccal plates , which 
are small and irregular and are scattered in the buccal membrane 
which surrounds the oral aperture. 
All these plates forming this singularly composite shell con- 
sist each of a network of calcareous spicula formed in the soft 
perisoma, and they thus have the same relation to the flesh of the 
echinus that the skeleton of a coral has to its investing body. 
The spicula meet and unite at right angles, and often form very 
definite patterns. 
In the living state the shell is covered with spines, each 
spine being articulated to one of the tubercles before spoken 
of. The articulation is effected, at least in the larger spines, 
by means of a ligament which passes from a little pit on the 
surface of the projecting tubercle to another little pit situated 
on the convex lower end, or base, of the spine, like the liga- 
mentum teres of our own hip joint. Thus great freedom of 
motion is allowed, while the motions themselves are effected 
by means of small muscles which extend from the shell around 
the tubercle to the projecting parts of the base of the spine. 
These muscles are, of course, invested by the external layer of 
the soft perisoma of the body. The spines are important organs 
of locomotion, and are the agents by which burrowing in the 
sand is effected. Locomotion, however, is also aided by tubular 
suckers, which abound in each ambulacrum. Each sucker springs 
from two pores, a minute tubular vessel passing out through 
each pore, and then each pair of such minute vessels joining to- 
gether to form one sucker. On the other hand, the two minute 
vessels unite again within the shell, to form a saccular dilata- 
tion, which is directly continuous with the ambulacral system 
of vessels to be presently described. 
Each sucker is mostly cylindrical, but is flattened and en- 
larged at its free end into a disk. This disk even is furnished 
with five flattened calcareous pieces, of about equal size, together 
forming the rosette , and the stalk of the sucker is often beset 
