86 



SPONGES. PHYLUM PORIFERA 



sponge, (b) by strengthening the wall, to make it possible for 

 sponges to grow to a greater size. 



SILICEOUS AND HORNY SPONGES 



The sponges of which we have so far spoken have calcareous 

 skeletons. A vast number of others have a skeleton of siliceous 



osc 



a 



I 



E 



Fig. 54. — Diagrams of the canal system of sponges. — Partly after Minchin. 



exhx., Exhalant canal; inh.c, inhalant canal; fi.c. flagellated chamber; osc, osculum ; osL, ostium; 



p., pore ; par., paragaster. 



(flinty) spicules. In all these the canal systems are of one of the 

 more complicated types, and usually they are made still more 

 intricate by ramifyings of the paragaster, and the appearance 

 of numerous oscula, which put it into communication with the 

 water at many points. The sponges of domestic use belong to a 

 comparatively small group in which the skeleton is not spicular 

 but a network of horny fibres, usually strengthened by sand 

 grains embedded in the fibres. Their canal system is of the type 

 which has small, round chambers, and in most of them these 



