92 SPONGES 



combination with siliceous spicules or foreign bodies ; or, finally, 

 sponges in which a skeleton is absent altogether. By means of 

 these various characters the Demospongiae are further subdivided 

 into a number of smaller groups. 



CLASS I. CALCAREA. 



The calcareous sponges are a very sharply denned group of 

 the Porifera. No forms are known in the remotest degree inter- 

 mediate between them and the other classes. As their name 

 implies, their chief characteristic is the possession of a skeleton made 

 up of calcareous spicules, a feature correlated with many other dis- 

 tinctive points of organisation and structure which render a cal- 

 careous sponge easy of recognition. 



From the point of view of evolution and morphology the 

 Calcarea are of special interest, since in all cases the starting- 

 point of the growth is the primitive vase -like Olynthus. The 

 characters of the adult sponge depend upon the particular manner 

 in which the Olynthus grows ; and calcareous sponges furthest 

 apart in the system differ, in the Olynthus stage, only in the 

 same trivial characters of spiculation or histology which are found 

 in the adult as specific distinctions. The Calcarea thus present 

 a most valuable and convincing demonstration of the theory 

 of evolution. Nevertheless, the powerful attraction and stimulus 

 which they offer to speculative and imaginative intellects has not 

 been without its drawbacks, for in scarcely any other group is the 

 classification and nomenclature in so confused a state ; and it might 

 almost be said that as many systems of the Calcarea have been 

 proposed as there are writers on the group. In spite, however, of 

 this diversity of opinion, no classification of the group has been 

 put forward as yet which can be considered in any way final ; and 

 the most fundamental problems of their phylogeny and natural 

 affinities are still in a very unsettled state. 



Canal System. Considered from the point of view of canal 

 system alone, the Calcarea are divisible into two grades. In the 

 first, the Homocoela or Ascons, are found the only known examples 

 of the first type of canal system (see above, p. 31). In the second, 

 the Heterocoela, corresponding to Haeckel's two families Sycons and 

 Leucons, the canal system is of the second or third type. Thus in 

 the Homocoela, as the name implies, the gastral layer is continuous, 

 i.e. the collar cells line the whole gastral cavity ; in the Heterocoela 

 it is discontinuous and restricted to the so - called flagellated 

 chambers. 



(a) The Canal System of the Homocoela. In the Ascons the 

 primitive Olynthus soon assumes a more complicated form, owing 

 to the growth of the body wall being localised chiefly in two 



