ECHINODERMA. ' 109 



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 

 cormnunicate 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 

 is 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 (falces) ; 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 is passed to the mouth. 



Echinoids live on seaweeds and the animals that are found on them ; 

 such as have no teeth, like Spatcmgus (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 larvse of larger 

 Crustacea. 



In a number of Echinoids and Asteroids some of the spines are 

 specially modified to act as seizing-organs — the free end being 



