OPHIUROIDEA OR BRITTLE-STARS. 209 



The largest are such as Asterias gigantea (from the Pacific coast of 

 N. America), measuring 2 feet in diameter, or Pycnopodia helianthoides, 

 about a yard in diameter, and with over twenty arms. 



There are many deep-sea forms, such as the ophiuroid-like Brisinga, 

 the widely distributed Hymenaster, and the blue Porcellenaster cceruleus, 

 but the majority occur in water of no great depth. 



In connection with external form, the many-rayed Solaster, the pin- 

 cushion-like Goniaster, the pentagonal pancake-like Palmipes, should 

 be considered. 



Parental care is incipient among Asteroids, for a large Asterias has 

 been seen sheltering its young within its arms ; there is a definite brood 

 pouch in the form of a sort of tent on the dorsal surface of Pteraster, 

 and there are other arrangements which serve a similar purpose. 



Many Asteroids break very readily, or throw off their arms when 

 these are seized. Professor Forbes describes how a fine specimen of 

 Luidia thus escaping him gave a wink of derision as it passed over the 

 side of the boat. The lost parts are slowly regenerated, and strange 

 forms are often found in process of regrowth. Thus the " comet -form " 

 of starfish occurs when a separated arm proceeds to grow the other four. 

 Asteroidea occur in Silurian strata. 



Class Ophiuroidea. Brittle-stars. 



The body of a brittle-star differs from that of a starfish in 

 the abruptness with which the arms spring from the central 

 disc. These arms are muscular, and useful in wriggling 

 and clambering ; they do not contain outgrowths of the gut, 

 nor reproductive organs. Moreover there is no ambulacral 

 groove, and the tube-feet which project on the sides are too 

 small to be of locomotor service. The madreporic plate is 

 situated on the ventral surface, usually on one of the plates 

 around the mouth. The food-canal ends blindly. 



The reproductive organs lie in pairs between the arms, 

 and open into pockets or burs» formed from inturnings of 

 the skin. The slits by which these bursse open are evident 

 at the bases of the arms. Water currents pass in and out, 

 perhaps aiding in respiration. 



The free-swimming larva is a Fluteus, very like that of 

 the Echinoids. 



Ophiuroids are first found in Silurian strata. 



I. Euryalida. Skin without plates, arms simple or branched 

 and capable of being rolled up. 

 Astrophyton. Gorgonocephalus. 



