70 ECHINODERMATA. ASTEROIDEA. 



ones. The Encrinasteroidea are confined to the Palaeozoic. 

 A few Euasteroids occur in the Devonian and Carbon- 

 iferous, but they are mainly Neozoic forms. The star- fishes 

 occur first in the Bala Beds, where we get the genus 

 Palceaster. They are more abundant in the Silurian, 

 where Palceaster, Palceasterina, and Palceocoma occur. 

 Palceaster and other forms are found in the Devonian ; in 

 the Jurassic, Uraster, Solaster, Astropecten, Plumaster, 

 Oreaster and Goniaster. The two last also occur in the 

 Cretaceous. In the Cainozoic rocks star-fishes are rare. 



CLASS. OPHIUROIDEA. 



In the Ophiuroids or brittle-stars, the body consists 

 of a disc and arms. The arms are generally five in 

 number, usually simple, but in some cases branched ; they 

 are much smaller than in the star-fishes and are merely 

 appendages of the disc and not prolongations. Usually 

 they are long, cylindrical and very flexible, serving for 

 locomotion by means of movements which take place 

 chiefly in a horizontal direction. Unlike the arms of the 

 star-fish they do not contain any prolongations of the 

 alimentary canal, and the ambulacral groove is not open 

 to the exterior. The arrangement of the nervous, the 

 water-vascular, and the vascular systems is similar to that 

 found in the Asteroidea; but the tube-feet are not pro- 

 vided with sucking-discs and there are no ampullae. There 

 is no anus and pedicellariae are absent. 



Generally there is a well-developed skeleton, but in 

 some cases granules only are found. In a typical case the 

 following parts are found. Encasing the arms, there are 

 four rows of plates (fig. 17), one dorsal, one ventral and 

 two lateral. The lateral are provided with rows of spines. 



