ECHINODERMS {SPINY ANIMALS) 



35 



Fig. 54. — Limy Plates 

 in portion of a ray. 



Fig. 55. — Starfish (showing 

 Madreporite). 



bilateral symmetry? The skeleton consists of limy plates embedded 



in the tough skin (Fig. 54). Is the skin rough or smooth ? Hard 



or soft? Are the projections (or spines) 



in the skin long or short? The skin is 



hardened by the 

 limy plates, ex- 

 cept around the 

 mouth, which is 

 at the centre of 



the lower side and surrounded by a mem- 

 brane. Which is rougher, the mouth side, 

 (orai side) or the opposite (aboral side) ? 

 Which side is more nearly flat ? The 

 vent is at or near the centre of the 

 disk on the aboral surface. It is usually 



very small and sometimes absent. Why a vent is not of much 



use will be understood after learning how the starfish takes food. 

 An organ peculiar to animals of this 



branch, and called the madreporic plate, 



or madreporite, is found on the aboral 



surface between the bases of two rays 



(Fig. 55). It is wartlike, and usually 



white or red. This plate is a sieve ; the 



small openings keep out sand but allow 



water to filter through. 



Movements : the Water-tube System. 



— The water, which is filtered through 



the perforated madreporite, is needed 



to supply a system of canals (Fig. 56). 



The madreporite opens into a canal 



called the stone canal, the wall of which 



is hardened by the same kind of mate- 

 rial as that found in the skin. The stone 



canal leads to the ring canal which sur- 

 rounds the mouth (Fig. 56). The ring 



canal sends radial canals into each ray to supply the double row 



of tube feet found in the groove at the lower side of each ray 



(Fig. 57). Because of their arrangement in rows, the feet are 



Fig. 56. — Water tube 

 System of starfish. 



m, madreporite; itc, stone 

 canal; ap, ampulla. 



