PHYLUM AND CLASS PORIFERA 



109 



>llared cells, but in many, as in Sycon, these cells are found 

 only in the radial canals, and not in the paragastric cavity, 

 which is lined with flattened cells like those of the ectoderm. 

 Sponges similar to Sycon gelatinosum, but with branching flagel- 

 late canals (Fig. 77, C), afford us the next grade of advancing 

 complexity. In these the incurrent as well as the flagellate 

 canals may form a branching system. In all the higher groups 

 of Sponges (Fig. 77, D, and Fig. 78) the flagellate endoderm 

 cells are confined to certain special enlargements of the canals 

 the so-called " ciliated chambers " C and the rest of the canals are 

 lined by flattened cells. 



Special names have been applied to the main types of canal- 

 system briefly sketched above. Forms in which the paragastric 

 cavity is lined by flagellate cells are said to belong to the Ascon 



PG 



L GC 



} . 78. Vertical section of a fresh-water sponge (Spongilla), showing the arrangement of the 

 canal-system. C. ciliated chambers ; DP. dermal pores ; Ex. excurrent canals ; GO. openings 

 of the excurrent canals ; PG. paragastric cavity ; SD. subdermal cavities ; 0. osculum. 

 (Modified from Leuckart and Nitsche's diagrams.) 



type, whether the paragastric cavity communicates directly or by 

 flagellate canals with the exterior. Forms in which there is a 

 paragastric cavity lined by flattened c'ells, and a system of radially 

 arranged flagellate chambers, are said to possess the Sycon type of 

 structure. Such Sponges as have small rounded flagellate cham- 

 bers (" ciliated chambers "), communicating in most cases by 

 narrow branching incurrent canals with tha exterior (directly or 

 indirectly) on the one hand, and by similar excurrent canals with 

 the paragastric cavity on the other the flagellate cells being 

 confined to the flagellate chambers are said to possess the Eliagon 

 type of canal-system. 



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 gelatinosum, and this effect 

 is increased by the development of a variety of infoldings of the 



