HEXACTINELLIDA 



203 



96). The former set of genera constitute the sub-class Amphi 

 discophora, the latter 

 the Hexasterophora. 



Sub-Class 1. Am- 

 phidiscophora Am - 

 phidiscs are present, 

 hexasters absent. A 

 tuft of rooting spicules 

 or basalia is always 

 present. The ciliated 

 eh timbers deviate more 

 or less from the typical 

 thimble shape, and the 

 m e mbrana reticularis 

 is continuous from 

 chamber to chamber 

 (Figs. 94, 95,' 97). 



Fig. 95. Portion ol' body-wall of Hyalonema, in 

 section, showing the irregular chambers. 



Sub -Class 2. Hexastero- 

 phora. Hexasters are present, 

 amphidiscs absent. The cham- 

 bers have the typical regular form , 

 and are sharply marked off from 

 one another (Figs. 90, 96). 



All the Amphidiscophora 

 have Lyssacine skeletons ; in 

 the Hexasterophora both types 

 of skeleton occur. The sub- 

 division of the Hexasterophora 

 is determined by the presence 

 or absence of uncinate spicules. 

 An " uncinatum " is a diactine 

 spicule, pointed at both ends 

 and bearing barbs all directed 

 towards one end. This method 

 of classification gives us a 

 wholly Dictyonine order, 

 Uncinataria, and an order 

 consisting partly of Dictyo- 

 nine, partly of Lyssacine 

 genera, which may be distinguished as the ANUNCINATABIA. 



Flo. y(j. Hexasters. A, Graphiohexast<r 

 B. Horicome ; C, onychaster. 



