56 STARFISH GALLERY. 



In the body of the Starfish (fig. 86) the arms are seen to be 

 continuous with the disk and to contain portions or prolongations 

 of the chief organs. The middle of the arm is occupied by two 

 rows of hard pieces (ambulacral ossicles), the fellows of which make 

 an open angle with each other, and so form an open ambulacral 

 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 

 madreporite (fig. 35, 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 7 d) 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 

 supporting 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 

 groove on the arm is the line 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 6e), use their 



