MEGASCLERES. 



83 



c. Strongytc rounded at each end (9). 



d. Tylotc knob-like thickening at each end (10). 

 With dissimilar ends : 



e. Strongle-oxea oxeate externally. 



f. Tylotoxca ,, (Fig. 72, 12). 



g. Oxystrongylc ,, internally. 



h. Oxytylote ,, ,, (Fig. 72, 11\ 



i. Oxydad externally ends in two or more secondary actines or cladi. 



k. Strongyloclad ectactine cladose. 



1. Tylochid ,, ,, 



The two latter have generally three cladi, in which case we have the character- 

 istic spicule of the Tetractinellida, the Triaene. 



The Triaene consists of the rhabdomc, or shaft, and the cladome, which consists 

 of the three cladi, a straight line joining the ends of the two cladi is the chord. 

 The sagitta is a perpendicular from the origin of the cladome to the chord. 



7 8 r 



11 



10 







Fio. 72. Megascleres of the Monactinellida and of the Tetractinellida. 1, plagiotrisene ; 

 2, 6, protrisene ; 3, 5, aiiatriane ; 4, clichotrisene ; 7, oxea ; S, tornote ; 0, strongyle ; 10, 

 tylote ; 11, oxytylote ; 12, tylotoxea ; 13, style ; Ik, 15, 16, clesmas (after Sollas, from Perrier). 



cladl directed forwards at different 

 the rhabclome. 



Wlth 



Varieties of Trisene : 



a. Anatricenc (anchor) cladi directed backwards (Fig. 72, 3, 5). 



b. Protricene (Fig. 72. 2, 6), 



c. Plagioirtene (Fig. 72, 1), 



, n .j . . \ 



d. Orthotnccne ) 



e. Dichotricene cladi dichotomous (Fig. 72, $) 



f. Trickotricene cladi trifurcate. 



g. Phyllotricene with lamellar cladi, found only in Lithistida. 



h. Discotricene cladome is a disc in which the separate cladi are not dis- 



tinguishable ; only in Lithistida. 



i. Amphitrice.ne both ends of rhabdome end iu a tritene. 

 k. Centrotricene the cladi arise from the centre of the rhabdome. 



