332 MANUAL OF NATURAL HISTORY. 



of the back, not continuous ; jaws compli- 

 cated ; ovaries five. 



II. SU3-01tDEIt.AKROWGiAXS (Akropygia). 



Mouth inferior, central ; vent superior, apical. 



4. FAMILY. Sea-Urchins (Echinidse). Body globose, 



sometimes depressed; ambulacra five, con- 

 tinuous from mouth to apex ; spiniferous 

 tubercles simple, rounded; spines generally 

 numerous, mostly equal sized ; ovaries five ; 

 dental apparatus complex. 



5. FAMILY. Turban -Urchins (Cidaridse). Body 



oblato- spheroidal ; ambulacra continuous , 

 from mouth to vent ; spiniferous tubercles 

 perforate ; spines often of different kinds, 

 usually long, unequal, seldom numerous ; 

 teeth complicated ; ovaries five. 



IV. ORDER. TRUE-STARFISHES (Cirrhigrada.) 



Form stellate or angular ; body covered with a 

 tough, leathery integument, more or less strength- 

 ened with a net-work of calcareous plates, and in 

 most with strong spines ; mouth central, inferior, 

 from which radiate to the extremities of the arms 

 grooves (ambulacra) containing extensile suckers. 



I. SUB-ORDER. DISTACHIAN-STARFISHES (Distachiata). 



Ambulacral grooves with two rows of suckers. 

 1. FAMILY. Membranous Sea-Stars (Asterinidse). 

 Body discoidal or pyramidal, mostly pent- 

 agonal, sharp edged ; skeleton formed of com- 



