6 ECHINODERMA. 
A Sea-Urchin is provided with a most effective comminuting organ 
in the shape of the pyramidal “ Lantern of Aristotle,” in which 
work five strong teeth, ending in a wedge like the teeth of a rat 
or other rodent. This Lantern is connected with a circlet of 
upstanding pieces placed just inside the margin of the mouth 
(actinostome of Agassiz), and consisting of the five “‘ auricles” as 
they are called. The dental apparatus may undergo reduction and, 
finally, in the Heart-Urchins and Spatangus it disappears alto- 
gether, and the creature is absolutely edentulous; such Urchins 
have the margin of the mouth produced into a spout, by means 
of which they bore in the sand in which they live and from which 
they extract what nutriment they can. 
The material which is undigested passes out by an anus in all 
but a few Starfishes and all Brittle-stars, which are aproctous. 
The anus is either exactly opposite the mouth and quite distinct, 
as in Holothurians and the regular Sea-Urchins ; or it shifts towards 
the margin and even on to the oral surface, as in the irregular Sea- 
Urchins ; or it is on the back, but not quite central and generally 
small, as in the proctuchous Starfish; or it is on the oral surface, 
near the mouth, and produced into an anal cone, as in Crinoids. 
Respiratory Organs.—The podia are both locomotor and respiratory 
in function; as they project from the surface of the body, have com- 
paratively delicate walls, and are, from time to time, filled with fluid, 
they may be supposed to be always respiratory in a general way ; 
but sometimes they are so fattened or pointed that they clearly can 
have no sucking action, and are purely respiratory. Special organs 
for respiratory purposes are not always developed, or, if so, are of 
a purely generalized character, as the soft membranous pouches or 
saccules which project from among the interstices of the calcareous 
skeleton of a Starfish. In an ordinary Sea-Urchin there may be 
seen on the membrane around the mouth (buccal membrane) five 
pairs of small folded processes, lodged in narrow slits of the margin 
of the test; these are known as gills and the slits as the gill-slits. 
In the Piper (Cidaris) these gills are wanting, but there are about 
the Lantern of Aristotle five large membranous sacs which are sup- 
posed to have a respiratory function ; they are known as the Organs 
of Stewart, after the name of their discoverer. In the Ophiuroids 
there is, on each side of the base of every arm, a slit which leads 
into a pouch or sac; into and out of this sac water passes, and as 
its walls are thin an exchange of gas is effected between the fluid 
in the body-cavity and the water which is pumped in and out; 
these sacs are known as bursal sacs, or burse. In Holothurians 
water may be, as in some species of Stichopus, pumped into and 
out of the intestine, or from the terminal portion of the digestive 
tract a trunk on either side may give rise to a more or less 
elaborately branched ‘respiratory tree” or system of fine and 
delicate tubes by means of which water carrying fresh oxygen 
comes into all but contact with the fluid in the body-cavity ; lastly, 
some Holothurians have no special respiratory organs, and have 
consequently been spoken of as Apneumona. 
