118 



ZOOLOGY 



SECT. 



but with flagellate canals arranged in groups, each group centred 

 round a main excurrent canal (Fig. 86, G) afford us the next 

 grade of advancing complexity. In these the incurrent canals may 

 form a branching system. In all the higher groups of Sponges 

 (Fig. 86, D and Fig. 87) the flagellate cells are confined to cer- 

 tain 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 

 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 cells, and a system of radially 



P,G 



Ex 



Fio. 87. — Vertical section of a fresh-water sponge (Spongllla), showing the arrangement of the 

 canal-system. C. ciliated chambers ; DP. dermal pores ; Ex. excurrent canals ; 60. openings 

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

 (Modified from Leuckart and Nitsche's diagrams.) 



arranged flagellate chambers, are said to possess the Syt'on type of 

 structure. Such Sponges as have small rounded flagellate cham- 

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

 narrow branching incurrent canals with the 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 Bhagon 

 type of canal-system. In the Ithagon proper the arrangement ox 

 parts is very simple. The Sponge has a paragastric cavity opening 

 on the exterior by an osculum. Opening into this central cavity 

 by wide apopyles are a number of rounded chambers each com- 

 municating with the exterior by an inhalant pore (prosopyle). 



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 01 

 zooids similar to the branches of Sycon gelatinosum ; and this effect 



