260 PHYLUM ECHINODERMA. 



forming the carbonate of lime skeleton, but facts ara 

 wanting. 



The sexes are separate, and they are like one another, 

 both externally and internally. The gonads develop periodi- 

 cally, and lie in pairs in each arm. Each is branched like 

 an elongated bunch of grapes, and is surrounded by a 

 "blood 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 in- 

 teresting sexual variability : many are first males and then 

 females (protandric), others are simply hermaphrodites, 

 others seem exclusively of one sex. The eggs of starfishes 

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

 known as a Bipinnaria or as a Brachiolaria. 



Other Starfishes 



Parental care is incipient among Asteroids. A species of 

 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 starfish 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 c<zruleus\ 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 pedicellariae sessile (if present), with two rows of 

 tube-feet. 



e.g. Astropecteri) Luidia, Porania, Asterina^ Palinipes. 

 Order II. Cryptozonia. With indistinct or rudimentary marginal 

 plates in the adults, often with intermediate plates between the 



