DEMOSPONGIAE 209 



spicule of the Heteractinellida is a euaster. Hence we may refer 

 the Octactinellid fossils to the class Hexactinellida, and the Heter- 

 actinellid forms either to the Monaxonida or Tetractinellida. 



CLASS III. DEMOSPONGIAE 



Silicispongiae in which triaxonid spicules are absent. 



This class has attained the highest level of organisation known 

 among Porifera ; the most efficient current-producing apparatus 

 is met with here, so, too, are protective coverings, stout coherent 

 skeletons, and the highest degree of histological differentiation 

 found in the phylum. 



Correspondingly it is the most successful group, the majority 

 of existing sponges coming within its boundaries. A few genera 

 and species are exceedingly specialised, for example, Bisyringa 

 dissimilis (p. 215). These, however, contribute only a very small 

 contingent to the Demosponge population, those species which are 

 really prolific and abundant being, as we should expect, the less 

 exaggerated types. 



Canal System.— With a few exceptions the representatives of 

 the Demospongiae may be said to have taken up the evolution 

 of the canal system at the stage where it was left in Leucandra 

 aspera — a stage which the ancestral Demosponges must have 

 reached quite independently of the Calcarea. These commoner 

 members are thus already gifted with the advantages pertaining 

 to a spherical form of ciliated chamber, and so, too, is the Ehagon 

 (Fig. 105), an immature stage noteworthy as the simplest form 

 of Demosponge, and thus the starting-point for the higher types 

 of canal system. The exceptions above alluded to are not with- 

 out interest : they are the Dendroceratina, of doubtful affinities, 

 (p. 220), which possess small tubular Syconate chambers. They 

 may be regarded either as of independent origin from other 

 Demospongiae, thus making the group polyphyletic, or more 

 simply as representing the ancestral condition, and in this case 

 we must look on the possession of spherical chambers by the 

 Ehagon as a secondary feature. Occupying as it does the 

 important position above indicated, the Ehagon merits a brief 

 description. It is a small discoid or hemispherical body attached 

 by a flat base. It contains a central paragaster, with a single 

 osculum at the free end. Into the paragaster open directly a 



VOL. I ^ 



