A SPONGE “99 
- The substance of which the sponge is composed is silky 
in nature—hence its strength and softness. The silky 
nature of the material would suggest. its animal origin, for 
we know that silk is an animal product being made by 
certain kinds of insects and by spiders. Sponges were 
originally regarded as belonging to the vegetable kingdom, 
Fig. 67.—Structure of common Sponge diagrammatically represented. 
a, sponge-flesh; b, pores opening into canals; ¢, ciliated chambers; d, large exhalent 
canals; e, central cavity. The canal system is black, and the course of the water- 
currents is indicated by arrow heads. 
but are now universally admitted to be animals, though 
unlike most animals we are acquainted with, they are 
unable to move about. 
The sponge as we have seen is composed of a fibrous 
substance in which are numerous rounded chambers. 
These are connected with the outer surface by a network 
