ECHINODERMATA. 75 



Class II. Asteroidea. Star-fish. 



Body star-shaped or pentagonal; the tube-feet on the ventral surface. 



Order 1. Stellerida. Digestive caeca, extending into the arms; anus 

 dorsal when present. Tube-feet situated in ambulacral grooves, usually 

 end in suckers ; maclreporic plate dorsal and interradial. Some develop 

 directly in a brood-cavity of the mother, the rest with a metamorphosis 

 (Bipinnaria, Brachiolaria). PedicellariaB. Asterias (Uraster) rubens, 

 common star-fish, and A. glacialis, tube-feet in four rows. Solaster 

 papposus, sun-star, generally with thirteen arms. Astropecten auran- 

 tiacus, tube-feet without suckers, in two rows. Asterina gibbosa, 

 pentagonal. 



Order 2. Ophiurida, brittle stars. Arms cylindrical, sharply marked 

 off from body; stomach without caecal prolongations. Ambulacral 

 furrows covered in by dermal plates, so that the tube-feet, which are 

 devoid of suckers, protrude at the sides of the arms. No anus ; no pedi- 

 cellarise (as a rule). Madreporic plate ventral, usually fused to an oral 

 plate. Larval form a Pluteus. Ophiothrix fragilis and Amphiura 

 squamata, viviparous. Astrophyton arborescens with soft-skinned 

 ventral surface, branched arms, and pedicellarise. 



