IIO ANNULOIDA. 



9. Echinida. Test regular, spheroidal. Mouth central ; 

 anus opposite the mouth, surrounded by the genital disc. 

 Ambulacra wide or narrow, usually wide centrally, diminish- 

 ing towards the mouth and anus. Mouth toothed. Spines 

 varied in length and shape. 



Section a. Salenida. Apical disk of more than ten 

 plates, large, with the anus placed excentrically in it. 

 Distrib. Oolitic to Tertiary. III. Gen. Salenia (fig. 62), 

 Acrosalenia. 



Section b. Diademadce. Ambulacral areas with two or 

 four rows of large tubercles. Distrib. Jurassic to Recent. 

 ///. Gen. Diadema, Astropyga. 



Section c. Hemicidarida. Interambulacra with two 

 rows of large tubercles. Distrib. Triassic to Cretaceous. 

 III. Gen. Hemicidaris (fig. 60). 



Section d. EcJunidce Proper. Ambulacral and inter- 

 ambulacral areas with large tubercles. Distrib. Jurassic 

 to Recent. III. Gen. Echinus, Temnopleurus. 



10. PeriscJioechinidcz. Test regular. Mouth central, infe- 

 rior ; anus opposite to the mouth, surrounded by the genital 

 disc. Ambulacra simple, perfect. Corona composed of more 

 than twenty rows of plates, each interambulacral area consist- 

 ing of from three to six rows. Distrib. Upper Silurian to 

 Carboniferous. ///. Gen. Archceocidaris and Palachinus. 



CHAPTER XI. 



ASTEROIDEA AND OPHIUROIDEA. 

 ORDER II. ASTEROIDEA. 



THE order Asteroidea or Stellerida comprises the ordinary 

 " star-fishes," and is defined by the fact that the body (fig. 65) 

 is star-shaped or pentagonal, and consists of a central " disc" 

 surroimded by five or more lobes or " arms." The arms are 

 truly prolongations of the body, are hollow, and contain prolonga- 

 tions of the stomach in their interior. The arms are, further, 

 grooved on their under surface for the reception of the ambulacral 

 or water-vascular vessels. From these grooves the tube-feet are 

 protruded in two or four rows. The integument (perisome} is 

 leathery, but is more or less calcified by the development in it of 

 plates, granules, and spines of carbonate of lime. The mouth is 



