CHAPTER VIII 



POEIFERA {continued) : FORMS OF SPICULES CALCAEEA 



HOMOCOELA HETEKOCOELA HEXACTINELLIDA DEMOSPON- 



GIAE TETRACTINELLIDA MONA XONID A CERATOS A KEY 



TO BRITISH GENERA OF SPONGES 



Sponges fall naturally into two branches differing in the size of 

 their choanocytes : in the Megamastictoka these cells are rela- 

 tively large, varying from 5//, to 9/i in diameter ; in Micromas- 

 tictora they are about 3/i in diameter.'' Por further subdivision 

 of the group the spicules are such important weapons in the 

 hands of the systematist that it is convenient to name them 

 according to a common scheme. This has been arrived at by 

 considering first the number of axes along which the main 

 branches >of the spicules are distributed, and secondly whether 

 growth has occurred in each of these axes in one or both directions 

 from a point of origin.^ 



I. Monaxons. — Spicules of rod-like form, in which growth is 

 directed from a single origin in one or both directions along a 

 single axis. The axis of any spicule is not necessarily straight, 

 it may be curved or undulating. The ray or rays are known as 

 actines. 



Biradiate monaxon spicules are termed " rhabdi " (Fig. 76, a). 

 A rhabdus pointed at both ends is an " oxea," rounded at both 

 ends a " strongyle," knobbed at both ends a " tylote." By 

 branching a rhabdus may become a "triaene" (Fig. 110, k, /). 



Uniradiate monaxon spicules are termed " styli." 



II. Teiraxons. — Spicules in which growth proceeds from an 



^ Sollas, Encyclopcedia Britannica, art. "Sponges," 1887. 



= Sollas, Ann. Mag. Nat. Hist. (5) iii. 1879, p. 23 ; Challenger Report, vol. xxv. 

 pt. Ixiii. 1888, p. Hi. 



183 



