OPHIUROIDEA OR BRITTLE STARS. 235 



Other Starfishes. 



Astropecten and most forms related to it have blind food canals ; 

 Brisinga has 9-12 long arms, arising abruptly from a small disc as in 

 Brittle stars, and has no ampullae, eye spots, or skin gills ; Luidia has 

 three-bladed pedicellariie ; in most forms the genital ducts end on plates 

 with a single aperture, and so on. 



The commonest European forms are species of Asterias or Aster- 

 acanthion, Astropecten, Cribrella, Solaster, Goniaster. 



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 ccertileus, 

 but the majority occur in water of no great depth. 



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 Pleraster. 



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 first occur in Silurian strata. 



Class OPHIUROIDEA. Brittle stars, e.g., the common 

 Ophiopholis bellis. 



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

 the abruptness with which the arms spring from the central 

 disc (cf. Brisinga). 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 madre- 

 poric 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 bursse formed from inturnings of 

 the skin, which communicate with the exterior by slits 

 opening at the bases of the arms. Water currents pass 

 in and out of these pockets, which probably have both 

 respiratory and excretory functions. 



