SPONGES 125 



[Cret.]. Family 10. *Vextriculitidae, Hinde. Genera — Pachyteichisma, 

 Z. [Jur.] ; Tfochobolus, Z. [Jur.] ; Phlydenium, Z. [Jur.] ; Ventriculites, 

 Mant. [Cret.], (Fig. 23); Schixorhabdus, Z. [Cret.]; Rhizopoterion, Z. 

 [Cret.] ; Sporadoscinia,Vom.\Cv&i.'\ ; Coeloscy2)hia,Tate [Cret.] ; Sestrocladia, 

 Hinde [Cret.] ; Lic)iiosi7iiou, Pom. [Cret.] ; Pobjblastidiiun, Z. [Cret.] ; 

 Ciphalitcs, T. Smith [Cret.]. 



CLASS III. DEMOSPONGIAE. 



The sponges included in this class appear at first sight a very 

 heterogeneous collection. The variations of structure are very 

 great, and between the Demospongiae -which stand furthest apart 

 in the scale — the Tetractinellids on the one hand, and the Keratosa 

 on the other — the differences are so pronounced that, if considered 

 by themselves, the former might be thought to have less in 

 common "with the latter than -with, for example, the Hexactinellids. 

 But even between extremes such as these, there is to be found a 

 complete series of intermediate forms, -which is nowhere interrupted 

 by any such abrupt distinctions as those -which mark off" the Demo- 

 spongiae as a whole from the other siliceous sponges. 



The Demospongiae represent, in fact, the class of sponges which 

 is the most widely spread, and most dominant at the present day, 

 comprising all the most familiar examples of the phylum Porifera. 

 Their cosmopolitan distribution places them amidst the most varied 

 conditions of existence, and they respond to the differences of their 

 environment by a wide range of adaptations. The Demospongiae 

 are at once the most plastic and the most highly organised of 

 sponges, as regards histological differentiation or elaboration of 

 anatomical structure. We find here the most perfect types of 

 canal system, and in such a form as Disijiinga (Fig. 26), with 

 its single incurrent aperture, we find the extreme of individualisa- 

 tion seen in any sponge. On the other hand, those Demospongiae 

 inhabiting the shore-line tend to lose their individuality, and to 

 advance towards an impersonal condition, in which the primitive 

 individual becomes merely an ill-defined physiological centre in a 

 spreading and often amorphous growth. 



Canal System. — The starting-point of the post-embryonic growth 

 and development in Demospongiae is a form known as the liharjon, 

 which, like the Olijnthus of Calcarea, represents a transitory stage 

 from which the existing forms of canal system in this group can be 

 derived by simple processes of growth. Hence the canal system 

 of the groups included under the designation Demospongiae — 

 the Tetractinellida, Monaxonida, Keratosa, etc. — are often known 

 as the Rhagon type of canal system. 



The Rhagon (Figs. 61, ^, and '^\) is a little sponge organism, in 



