SEA-URCHINS 219 
kinds, and proceed from the knobs on the exoskeleton, over 
which they fit, forming ball-and-socket joints, which enable them 
to move in any direction. The long spines are ribbed, and seem 
to have no other function than that of protection. The second 
set, the pedicellarie, are very peculiar small organs scattered over 
the surface in great numbers, and consisting of a head bearing 
three bill-like blades mounted on a long, flexible stalk. The office 
Test of sea-urchin, with the spines removed to show the arrangement of the 
plates, lateral view: Amb, ambulacral zone with its perforated plates; 4p, apical 
(aboral) pole; Int. amb., interambulacral zones. (From Bronn’s ‘' Thierreich.”) 
of some of the pedicellariee is to remove waste from the excretory 
opening; this is passed down regular lines and dropped into 
the water, thus keeping the body clean. Others are constantly 
opening and shutting their forks, reaching in all directions and 
grasping and removing anything which may have become entan- 
gled in the spines. They also capture floating bits of seaweed, 
which they drag over the body to conceal it. A third set of 
projections are the spheridia, small globular bodies said to be 
connected with perception. 
The spines vary greatly in size, number, and form in different 
species, and are such conspicuous features as to make the identi- 
fication of species easy. In some the spines are solid and fluted; 
