CHAPTER XIV. 



THE STELLEROIDEA. 1 



CLASS II. STELLEROIDEA. 



SUB-CLASS 1. ASTEROIDEA. 



Order 1. Phanerozonia. 

 2. Cryptozonia. 



SUB-CLASS 2. OPHIUROIDEA. 



Order 1. Lysophiurae. 



2. Streptophiurae. 



3. Cladophiurae. 



4. Zygophiurae. 



THE class of the Stelleroidea includes the starfish, brittle stars, 

 sand stars, basket-fish, and branching stars, all of which are 

 characterised by a depressed stellate body composed of a central 

 disc, whence radiates a number of rays or arms. They have 

 radiately arranged genital organs (i.e. are actinogonidial) ; they are 

 not attached by the aboral surface, nor is the oral surface furnished 

 with ciliated food-grooves (i.e. are eleutherozoic) ; and they usually 

 have the podia limited to the lower half of the body (i.e. are 

 lysactinic), instead of having them continued upward to the apical 

 plates, as in typical sea-urchins (which are desmactinic). The 

 radial water-vascular vessels lie along the under sides of the arms, 

 and are exterior to the ambulacral ossicles. The aperture of 

 the water-vascular system or madreporite is not connected with 

 the apical plates, as it is in all the Echinoids except Echinocystis. 



This list of characters is quite sufficient to mark off the 

 Stelleroids from all other Echinoderms. The class is divided into 

 two groups the Asteroidea and Ophiuroidea each of which is 

 usually ranked as a distinct class, but no definite line of separa- 

 tion can be drawn between them. The two characters on which 

 reliance is generally placed are the presence of an ambulacral 



1 By J. W. Gregory, D.Sc. MS. closed at end of 1896. 



