230 ECHINODERMA. 



plate, between the bases of the arms on the dorsal surface. 

 In Asterina gibbosa, however, the eggs are extruded 

 ventrally. In the same species there is an interesting 

 sexual variability : many are first males and then females 

 (protandric), others are simply hermaphrodites, others 

 seem exclusively of one sex. The eggs of star-fishes 

 are fertilised in the water, and the free-swimming larva is 

 known as a Bipinnaria or as a Brachiolaria. 



Other Star-fishes. 



The commonest European forms are species of Asterias or Astera- 

 canthion, Astropecten, Cribrella, Solaster, Goniaster. 



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

 Brisinga has nine to twelve long arms, arising abruptly from a small 

 disc, as in Brittle-stars, and has no ampullse, eye spots, or skin-gills ; 

 Luidia has three-bladed pedicellaris, and a very remarkable larva. 



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

 the widely-distributed Hymenaster, and the blue Porcellenaster caruleus ; 

 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 Pteraster. 



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

 these are seized. The lost parts are slowly regenerated, and strange 

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

 of star-fish occurs when a separated arm proceeds to grow the other four. 

 Asteroidea first occur in Silurian strata. 



Class Ophiuroidea. Brittle-stars, e.g. Ophiopholis bellis. 



The body of a brittle-star differs from that of a star-fish 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 



