MANUAL OF NATURAL HISTORY 333 



pressed, imbricate plates ; dorsal wart single, 

 rarely double. 



2. Family. — Porous Sea -Stars (Pentacerotidae). 



Body mostly pentagonal or sub-orbicular, in 

 some discoidal, supported by roundish or 

 elongated pieces, covered with a smooth or 

 granular skin, pierced with minute pores 

 between the tubercles ; rays mostly short, 

 but in some elongated. 



3. Family. — Tubercular Sea-Stars (Astropectini- 



dse). Stellate or multi-radiate ; back flattish, 

 netted with numerous tubercles, crowned 

 with radiating spines at the tip, named 

 Paxilli ; arms mostly more or less rounded. 



II. S UB-ORDER — Tetrastachian-Starfishes 

 (Tetrastachiata). 



Ambulacral grooves with four rows of suckers 



4. Family. — Sea-Stars proper (Asteriidse). Kays 



generally five, usually rounded, elongate, 

 sometimes spiny ; dorsal wart simple. 



V. ORDER. — Spinous Starfishes (Spinigrada). 



No branchial membranes ; body covered with 



calcareous scales or plates ; form regularly radiate ; 



true arms, clothed with spines by which motion is 



effected. 



1. Family. — Brittle-Stars (Ophiuridse) Body orbi- 

 cular, depressed, covered with spines or scales ; 

 arms mostly five, simple, with cross series of 

 spines on the sides ; mouth inferior, central. 



