CHAPTER XII. 



THE EDRIOASTEROIDEA. 1 



CLASS IV. EDRIOASTEROIDEA, E. BILLINGS (1854,-58; 

 HUXLEY, 1877; and BATHER, 1899) 



( = THYROIDA, Chapman, 1860; AGELACRINOIDEA, S. A. Miller, 

 1877-83 ; Worthen, 1883; CYSTASTEROIDEA, Steinmann, 1888; 

 F. Bernard, 1893; THECOIDEA, Jaekel, 1895). 



Not divided into Orders. 



PELMATOZOA in which the theca is composed of an indefinite 

 number of irregular plates, some of which are variously differen- 

 tiated in different genera ; with no subvective skeletal appendages, 

 but with central mouth, from which there radiate through the 

 theca five unbranched ambulacra, composed of a double series of 

 alternating plates (covering-plates), sometimes supported by an 

 outer series of larger alternating plates (side -plates or flooring- 

 plates). Pores between (not through) the ambulacral elements, 

 or between them and the thecal plates, permitted the passage of 

 extensions from the perradial water-vessels. Anus in posterior 

 interradius, on oral surface, closed by valvular pyramid. Hydro- 

 pore (usually, if not always, present) between mouth and anus. 



Reducing the characters of the Edrioasteroidea to their simplest 

 expression, one may imagine a schematic type of the following 

 nature : A flexible theca of sack form, composed of numerous 

 irregular, polygonal plates deposited in the integument, probably 

 with the stroma- strands between them still plainly visible (cf. 

 Stromatocystis) ; it would have a mouth in the centre of the upper 

 surface, and would be attached by some indefinite portion of the 

 lower surface ; the anus, with its valvular covering, would lie on 

 the upper surface, and there would probably be a hydropore 

 between it and the mouth. So far this type would present primi- 

 tive characters like those of the earlier Amphoridea, from which 



1 By F. A. Bather, M.A. 



