RADIATA. ECHINODERMATA. 473 



I.— ECHINODERMATA. 



Order I. — Free. 



Sect. I. — Locomotion executed by means of suckers. These, 

 consist of tiibulm- extensile jjroccsses, with a terminal, usually 

 fringed disc, which serves either as an organ of motion arrest; 

 the central canal admitting water to the aerating organs. 



A. Covering of immoveable testaceous plates, without projecting 

 arms. Echinid^. 



A A. Covering crustaceous and moveable. 



a. Body produced. Fistulidm. 



a a. Body depressed, or orbicular, and divided into arms or 

 rays. Asteriada. 



Sect. II. — Destitute of suckers for locomotion. 



Sipunculus. 

 Priapulus. 



Ordeu II. — Fixed ; the body covered by articulating plates, 

 supported by an articulated column. 



Crinoidae. Margin of the oral disc supporting numerous articu- 

 lated arms. 



Blastoidae. Margin of the oral disc destitute of arms ; body ^vith 

 avenues of branchial pores. 



