SEA-URCHINS. 
3°9 
JAWS of stone-urchin (nat size). 
rounding the mouth-opening, through which may be seen five pointed teeth. 
These belong to a very elaborate masticating apparatus, shown in the illustration, 
and found in all the regular urchins, as also in the Clypeastrida among the 
irregular urchins. This consists of twenty principal pieces arranged into a five¬ 
sided conical mass, compared by Aristotle 
to a lantern (a). In the centre of the 
whole are five teeth (b, c ), working in 
bony sockets, or pyramids, connected by 
muscles with one another, with the interior 
of the test, and with the arched processes, 
known as auricles ( d ), that surround the 
mouth - opening. There are yet other 
calcareous pieces connecting the pyramids 
together, and serving as attachments for yet other muscles. Such a sea-urchin 
as that described, preserves as much as any echinoderm the five-rayed symmetry 
of the group; but in many forms the five-rayed type is not so obvious, for the 
animal has become elongated along one of the axes, so as to have a superficial two- 
sided symmetry. This is naturally connected with constant movement in one 
direction, as though the animal had a head and tail; and such modification is found 
among those urchins that live on muddy bottoms, and especially in those from 
considerable depths. Not only is the test elongated, but the mouth moves forward 
to the front margin, and the vent down¬ 
wards to the hinder margin, so as 
eventually to lie on the under instead 
of on the upper surface of the test. An 
earlier stage in this modification is 
shown in the illustration of the shield- 
urchin ( Echinarachnius ), and a fully 
developed one in the heart-urchin 
(. Brissopsis ), with its long tube-feet 
extended in the act of walking towards 
the left. These heart-urchins, as they 
move along through the sand and mud, 
scoop it up into their mouths, and pass 
it through the intestine, extracting on 
its passage such nutriment as the 
minute organisms it contains can afford. 
To enable them to scoop it up in this 
way, the hinder margin of the mouth 
is produced forwards in a kind of shovel shape, as is shown in the illustration of 
a Pourtalesia test from which the spines have been removed. These animals live 
at very great depths in the sea, and are the urchins most modified in this particular 
direction. Urchins of the heart-shaped type have short delicate spines, and move 
almost entirely by their long tube-feet, in the manner described; but the greater 
number of the regular urchins progress chiefly by the aid of their spines, which 
are much stouter, while the tube-feet often have the suckers very imperfectly 
SHIELD-URCHIN, FROM ABOVE (nat. size). 
