16 



The remaining groups of the Porifera have become con- 

 siderably modified from the primitive ancestor, and their 

 phylogeny is not very clear. The Myxospongiae, in which 

 no spicules are present, are probably the least differentiated, 

 but may have degenerated somewhat, as is shewn by the line 

 in the table sloping slightly downwards. The great group of 

 the Fibrospongise, in which an extensive skeletal apparatus, 

 formed of kerotose fibres, and generally of siliceous spicules 

 also, is developed, have diverged with a considerable amount 

 of differentiation from a still more advanced point, leaving 

 the Hexactinellidse (the vitreous sponges) as the termination 

 of the Porifera branch, and farthest from the main stem 

 of the table. The Fibrospongiae have split up into a large 

 number of smaller groups. 



The great series of the Coelenterata arose from Gastrea, 

 the sac-like ancestor, with its wall formed of two layers of 

 cells, and having a central cavity and a mouth opening (see 

 fig. 11). Some of the descendants of this form diverged 



Fig. 11. Gastmla stage in the development of a Metazoon; a, epiblast; c, hypoblast; 

 6i, blastopore; en, archenteron. 



from the primitive Sponges on the one hand, and from 

 the ancestors of the higher Metazoa on the other, and by 

 the development of tentacles or outgrowths from the body. 



