Introduction to A nimal Morphology. 1 3 7 



mentation, becomes pentagonal. Development proceeds as 

 in pluteiform larva?. In some, reproduction possibly occurs 

 by fission, thus explaining the occurrence of heteractinism. 



The following families are included : 



i. . \-:, racanthiidce proctuchous ; ambulacra four-rowed ; 



iicellaria? stalked, straight, valvate, ex. Asteracanthion, the 



imon orange starfish. 2. Astropectinida3 ambulacra two- 

 rowed ; back netted, paxillated ; pedicellarire usually sessile; 

 anus none, except in Archaster ; body pentagonal (Cteno- 



us), or long-armed, with one ventral spine-bearing series 

 of border plates (LI wydia), or with two (Astropecten, Archaster). 

 3. Oreastridx ambulacra two-rowed; skin rough, pierced by 

 numerous pores, flat on both sides (Astrogonium), or convex dor- 

 sally, often with large coarse eminences (Oreaster, Asteropsis). 

 Kchinaster has long conical or cylindrical arms, with numerous 

 pores ; Cribrella, also long-armed, has few pores in the naked 

 interspaces of the netted skin ; Solaster has ten or more 

 paxillated arms, and many pores ; Chaelasler is clothed with 

 bristles, Ophidiaster with granulated plates. 4. Astcrinidx 

 ambulacra as in last ; body discoidal or pyramidal, sharp- 

 edg< ton of imbricate segments; dorsal wart single, 



rina, Palmipes ; Culcita is thick, with 

 a leathery surface ; Pa-raster has five short thick arms. 



CHAPTER XXL. 



CLASS 2. Km I \< 'IDEA (AgdSStz). 



GLOI , :<1, or hc't'irl-shaped forms ; 



Bd, r i^rrrn in colour; with a prri- 

 <l-rnial, spin. --clad, ; 6-angled, 



in usually twenty nn-riilional 

 , t-ath of many (5-20 / ]>! a ]>>lr to 



Tli.-].] >sits of calcium carbonate, and 



sulph sv other salts, in a basis of nncl< 



