MEGASCLERES. 



83 



c. Strongyle rounded at each end (9). 



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

 With dissimilar ends : 



e. Strongle-oxea oxeate externally. 



f. Tylotoxea ,, ,, (Fig. 72, 12). '; 



g. Oxystrongyle ,, internally. 



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



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



k. Strongylodad ectactine cladose. 



1. Tylodad ,, ,, 



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 rhabdome, or shaft, and the cladome, which consists 

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

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



12 rt 



9 10 





(!) 



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

 #, G, protrisene ; 3, 5, anatrisene ; k, dichotrisene ; 7, oxea ; 8, tornote ; 9, strongyle ; 10, 

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



Varieties of Trisene : 



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



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



c. Plagioiricene (Fig. 72, 1), l cladi dil ' ected forwards at different angles with 



j the rhabdome. 



d. Orthotricene 



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



f. Trichotricene 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. Amphitricene both ends of rhabdome end in a trisene. 

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



