ECHINODERMA. 109 
groove ; along this we find the suckers, the water-canal that 
supplies them, the blood-vessel of the arm, and a nerve-cord. At 
the centre of the disk is the mouth. The ossicles at the sides of 
the arms bear spines, which vary in different species ; the surface 
of the back is supported by a network of hard pieces, and through 
the intervening spaces there project membranous pouches, which 
are respiratory in function. The modified plate on the upper 
surface opens into a tube by means of which the water-vessels 
communicate with the exterior ; this plate is known as the madre- 
porite (Fig. 2, m). 
The organs for masticating the food are most highly developed 
in the regular Echinoids, where the complex apparatus known as 
the " Lantern of Aristotle " is found (Case 38) to consist of five 
sets of pieces ; the tooth is strong and bevelled at its free end ; it 
s supported by triangular jaws on either side, a pair uniting and 
having the form of an inverted pyramid ; these alveoli are con- 
nected with their neighbours by oblong pieces (fakes) ; above these 
there are elongated bars, which are hinged on to the inner end of 
the falces and have their outer ends free. The whole lantern is 
connected to the test by muscles which pass from its sides to the 
auricles or upstanding pillars which lie round the mouth ; and, owing 
to this muscular apparatus, the teeth are capable of complicated and 
various movements. 
In the Ophiuroids the edges of the mouth-slits are provided with 
short spinous processes, varying a good deal in arrangement, but 
never having, apparently, any other function than that of a filtering- 
apparatus ; in the Starfishes the plates round the mouth have a sup- 
porting function only ; in Crinoids and Holothurians the mouth is 
unarmed ; the latter are often remarkable for a deposit of calcareous 
plates in the walls of the gullet, and in the former the grooves on the 
arms are the lines along which food comes to the mouth. 
Echinoids live on seaweeds and the animals that are found on them ; 
such as have no teeth, like Spatangus (Case 32), use their spout-like 
mouth to take up the sand and debris on which they move, and from 
which they extract some nutriment. Ophiuroids live on the smaller 
foraminifera ; Asteroids on dead fishes (as line-fishermen well know), 
oysters, and other molluscs, and even on specimens of their own 
particular species ; Holothurians on shell or coral debris and the 
minute organisms it contains ; and Crinoids on small tests of 
foraminifera and on the adults of small and larvae of larger 
Crustacea. 
