68 GUIDE TO THE FOSSIL INVERTEBEATE ANIMALS. 



Gallery and in them pores are clearly seen. Pores, however, also 

 occur in the food-grooves of a stalked form, Steganohlastus. 

 The food-grooves of Edrioaster (Fig. 31) closely resemble 

 those of a starfish, except that they, as well as the mouth, 

 are protected by covering-plates like those of crinoids. It is 

 possible that these curious forms may throw some light on 

 the origin of starfish. 



Since Edrioasteroidea are rare and of exceptional interest, 

 both British and foreign examples are exhibited together in 

 Table-case 30, and are supplemented by reproductions. 



Class ASTEROIDEA. 



On the further side of Table-case 30, the free-moving 

 Echinoderms begin with the starfishes, generally regarded as 

 constituting the simplest Class of Eleutherozoa. In a starfish 

 the body is either markedly five-sided in outline or is more 

 or less star-shaped, in which case it is said to consist of a 

 central " disc " extended into " arms," which vary in number 

 from 5 (e.g. the common cross-fish, Asterias) to over 40 (e.g. 

 the sun-star, Heliaster); the mouth is in the centre of the 

 body and is turned to the sea-floor ; the anus is almost in 

 the centre of the upper surface, but is absent in a few forms ; 

 the under side of the arms is grooved, and along each groove 

 runs a vessel of the hydraulic system ; this vessel gives off 

 side-branches which end in free processes (podia) differing 

 from those of Pelmatozoa in that each terminates in a 

 sucker; between the flooring-plates of the groove are pores, 

 through which pass branches from the podia, each com- 

 municating with a swelling (ampulla) within the body. This 

 arrangement of the podia enables each one to be extended 

 for locomotion, and to be withdrawn into the groove by the 

 passage of the fluid from it into the ampulla; such an 

 arrangement is found in no Pelmatozoa except perhaps some 

 Edrioasteroidea ; but from those forms starfish differ in 

 having the groove unprotected by covering-plates. The 

 remainder of the starfish skeleton consists usually of small 

 plates or bars which serve to strengthen and support the 

 stout but flexible skin. 

 Table-case The oldest Palaeozoic starfishes in the British collec- 

 tion are Uranaster and Palaeaster, represented by casts in 

 Caradoc sandstone of Upper Ordovician age. From the 

 Wenlock beds comes the heavily plated and many-armed 

 Lepidastcr. The Lower Ludlow shales of Leintwardine, 



