SPONGES 



139 



blasts, and consequently the amount of spongin, increases pari 

 passu with a decrease in the number of fibre cells, which tend to be 

 placed externally to the spongoblasts (cf. Fig. 92, A, B). Finally, 

 tlie spicules become wholly enveloped in spongin, the result being 

 a fibre of spongin containing a core of spicules, the whole enveloped 

 in a fibrous sheath (Fig. 92, C). A still further stage, in which the 



The evolution of a spongin skeleton as seen in types of Renieiinae and Chalininae and in 

 Kus])ongia. A. skeletal framework of lieniera : li, of PachychaUnit ; C, of Chalina: D, of 

 Euspongin. sp, spicules ; spg, spongin ; m.f, main fibres ; cf, connecting fibres ; spg.f, spongin 

 fibres ; 1:011, connlns. 



spicules in the interiur of the fibres atrophy and disappear (Fig. 92, 

 D), produces a type of sponge skeleton which can only be dis- 

 tinguished froTU that of the Keratosa by arbitrary definitions 

 (presence or absence of spicules outside the fibres). The place of 

 the spicules is taken in many cases by sand grains or foreign 

 particles of various kinds. 



There can, in fact, be found in the Monaxonida every possible 

 stage required for the phylogeny of the true horny sponges 



