m 



PHYLUM AND CLASS PORIFERA 



119 



flagellate canals arranged in groups, each group centred round a 

 main excurrent canal (Fig. 87, C), afford us the next grade of 

 advancing complexity. In these the iucurrent canals may form 

 a branching system. In all the higher groups of Sponges 

 (Fig. 87, D, and Fig. 88) the flagellate cells are confined to certain 

 special enlargements of the canals the so-called " ciliated cham- 

 bers " (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 

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

 structure. Such Sponges as have small rounded flagellate chani- 



c P , G ~ 



00 



fn 



EJ- 



FIG. 88. 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; /(?. paragastric cavity; S O. subdermal cavities;' 

 0. osculum. (Modified from Leuckart and Nitsche's diagrams.) 



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 Rhagon 

 type of canal-system. In the Rhagon proper the arrangement of 

 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). 



A thicker or thinner specialised outer layer the dermal cortex 

 situated immediately below the superficial ectoderm, is present 

 in many Sponges. This is a layer of mesogloea with special 

 skeletal elements, usually containing spaces and canals lined by 

 ectoderm (subdermal cavities, Fig. 88, SD) which communicate 



