298 PHYLUM ECHINODERMA 



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 

 (Fig. 161). 



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

 like Brisinga, the widely distributed Hymenaster^ and the 

 blue Porcellenaster coeruleus ; 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. Astropecten, Luidia, Porania, 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 

 pedicellariffi sessile or stalked (if present), often with apparently 

 four rows of tube-feet. 



e.g. Aster ias, Solaster, Henricia, Bnsinga. 



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



Echinoderms with a stellate flattened body, nearly related 

 to starfishes, but usually dijfering from them in having the 

 amis {sometimes branched) sharply marked off from the 

 central disc, no ambulacral groove on the ventral surface of 

 the arms, the digestive organs and gonads restricted to the 

 disc, and the madreporite ventral. There is no anus. There 

 are deep respiratory clefts on the disc at the insertion of the 

 arms. They agree with starfishes in being free, in having 

 radially disposed gonads, in having the tube-feet restricted to 

 the under surface, and in other features. 



The body of a brittle-star diff'ers from that of a star- 

 fish in the abruptness with which the arms spring from 



