CHAPTER VIII 



PORIFERA {CONTINUED) : FORMS OF SPICULES CALCAREA 



HOMOCOELA HETEROCOELA HEXACTINELLIDA DEMOSPON- 



GIAE TETRACTINELLIDA MONAXONIDA CERATOSA KEY 



TO BRITISH GENERA OF SPONGES 



Sponges fall naturally into two branches differing in the size of 

 their ehoanocytes : in the Megamastictoka these cells are rela- 

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

 tictora they are about 3/i in diameter. 1 For 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. 2 



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 t I). 



Uni radiate monaxon spicules are termed " styli." 



II. Teiraxons. Spicules in which growth proceeds from an 



1 Sollas, Encyclopaedia Briiannica, art. "Sponges," 1887. 



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

 pt. lxiii. 1888, p. Hi. 



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