268 
ON THE ECHINODERMATA. 
is keeled on its outer surface, and bordered by raised margins. The 
corresponding sides of the two contiguous pieces are bound together by 
strong muscles, and moved about in every direction by a complicated 
arrangement of cords and levers. Each of the contiguous sides are 
grooved like files, between which every particle of food must pass to 
be filed down, as in a mill, before it can enter the stomach; each jaw or 
socket carries a tooth which projects beyond the mouth. The tooth is 
developed from a pulp, which surrounds the root of the tooth, so that 
just as the tooth wears away at the point it is renewed at the root. 
The tooth has a prismatic form, and is never blunted bj^’ work. This 
apparatus in construction and uses is quite unique among animal 
structures, the jaws being destined to rub down all fragments upon 
which the urchins feed, so that the last crumbs which fall to the 
bottom of the sea, from Nature’s bounteous banquet, maybe all rubbed 
down and used up as nourishment by these lowly Echinoderms. The 
intestinal canal makes two and a half turns round the shell, and is 
always filled with sand and other debris from which nourishment is 
extracted, just as the earthworms in our soil are constantly passing 
the vegetable mould through their intestines and loosening the earth, 
nourishing their bodies at the same time. The blood moves in a true 
circulation of vessels, which are spread out upon the surface of the 
digestive organs and distributed throughout the body, forming the 
blood vascular system, in opposition to another set of vessels destined 
for the circulation of water throughout the body, called the water 
vascular system. The water enters freely through appropriate 
apertures in the shell and washes all the included viscera, giving out 
its oxygen to the blood and renovating the circulatory fluids; the 
water flows in currents created by the action of the cilia, which are 
developed on the living membrane of the shell, so that ciliary motion 
urges on the water, and constantly renews the streams that every¬ 
where meander throughout the interior of the calcareous box. The 
nervous system of Asterias and Echinus consists of a nervous ring 
which surrounds the gullet and develops knots or ganglia at each 
of the five divisions of the body. From these ganglia nerves 
proceed to the organs, and one long branch passes out to the end of 
the ray in the starfish and becomes its optic nerve, and in the 
Echinus the nerve passes up inside the shell and supplies the eyes 
placed in the apical disc. 
Holothuroidea have the body in general elongated, the skin usually 
soft and leathery, in a few genera strengthened by calcareous or horny 
spines ; five avenues of suckers, which divide the body into as many 
longitudinal nearly equal, lobes or segments ; the mouth is surrounded 
by plumose tentacula, the numbers of which are, in general, multiples 
of five; vent at the opposite extremity of the body. The digestive 
organs consist of a long intestine, which makes some coils in passing 
through the body. Kespiration is carried on by a singular arrange¬ 
ment of ramified tubes, like a miniature tree, rising from the cloaca, 
which inhales and exhales the water many times a minute. In this 
