ASTEKOIDEA. 247 



sinus. Each has a separate duct, which opens on a porous 

 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. 



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. 



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

 like Brisinga, the widely-distributed Hymenaster, and the 

 blue Porcellenaster cwruleus ; but the majority occur in 

 water of no great depth. 



Asteroidea first occur in Silurian strata. 



Classification.- 



Order I. Phanerozonia. With strongly developed marginal 

 plates, the upper and lower marginals in contact ; with skin- 

 gills restricted to the dorsal (abactinal) surface : with broad 

 ambulacral plates ; with prominent adambulacrals in the peri- 

 stome, with pedicellarice sessile (if present), with two rows of 

 tube-feet. 



e.g. Aslropecten, Lnidia. Paranja, Asterina, Palmipes. 



Order II. Cryptozonia. With indistinct or rudimentary marginal 

 plates in the adults, often with intermediate plates between the 

 upper and lower marginals, with skin-gills not restricted to the 

 dorsal (abactinal) surface, with narrow ambulacral plates, with 

 ambulacrals or adambulacrals prominent in the peristome, with 

 pedicellarire sessile or stalked (if present), often with four rows 

 of tube-feet. 



e.g. Asterias, Solaster, Henricia, Brisinga. 



