Introduction to Animal Morphology. 137 



Tnentation, becomes pentagonal. Development proceeds as 

 in pluteiform larvoe. In some, reproduction possibly occurs 

 by fission, thus explaining the occurrence of heteractinism. 



The following families are included : — 



I. Asteracanthiidce — proctuchous ; ambulacra four-rowed ; 

 pedicellarise stalked, straight, valvate, ex. Asteracanthion, the 

 common orange starfish. 2. Astropectinidae — ambulacra two- 

 rowed ; back netted, paxillated ; pedicellarise usually sessile ; 

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

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

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

 3. Oreastridae — 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). 

 Echinaster 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 ; Chaetaster is clothed with 

 bristles, Ophidiaster with granulated plates. 4. Asterinidse — 

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

 edged ; skeleton of imbricate segments ; dorsal wart single, 

 rarely double, ex. Asterina, Palmipes ; Culcita is thick, with 

 a leathery surface ; Pteraster has five short thick arms. 



CHAPTER XXI. • 



CLASS 2. ECHINOIDEA [Agassiz). 



Globular, oval, discoid, or heart-shaped forms ; 

 brown, orange, red, or green in colour ; with a peri- 

 some (corona) of dermal, spine-clad, 4-6-angled, cal- 

 ■careous plates, arranged in usually twenty meridional 

 rows, each of many (5-20) plates passing from pole to 

 pole. The plates are deposits of calcium carbonate, and 

 sulphate and a few other salts, in a basis of nucleated 



