I" PHYLUM PORIFERA 85 



paragastric cavity, consists throughout of flagellate collared 

 cells similar to those of the flagellate canals of Sycon. 



The majority of sponges, however, are more complicated 

 in structure than Sycon. One of the causes of their 

 complexity being that the canals, instead of being simple 

 and straight, become branched, forming a system, often 

 highly complicated, of ramifying channels. In these more 

 complex sponges the flagellate collared cells are confined to 



P , G 



Ex 



Fig. 38. — Vertical section of a fresh-water sponge (Spongilla), showing the arrange- 

 ment of the canal-system. C, ciliated chambers; DP, dermal pores; Ex, 

 exenrrent canals; GO, openings of the excurrent canals; PG, paragastric 

 cavity; SD, subdermai cavities; O, osculum. (Modified from Leuckart and 

 Nitsche's diagrams.) 



certain rounded dilatations of the canals — the flagellate 

 chambers. 



Moreover, in the more complex forms the development 

 of branches from the originally simple sponge, and the 

 coalescence of neighbouring branches with one another, 

 greatly obscure the essential nature of the sponge as a 

 colony of zooids similar to the branches of Sycon ; and 

 this effect is increased by the development of a variety of 

 infoldings of the ectoderm which appear in the higher 

 forms. 



