158 



INTRoDUCrroN TO ZfloLfiGY. 



arai.mnt and only sufficient to hold togcthrrtlie spicules, 

 wliicli are very numerous. 



There aretlireo kiiu^s of spicules in a fresh-water 

 sponge : skeleton sjhcules, dermal spicules, and gem- 

 mule spicules. 



Microscopic Objects. 



Tlie skeleton spicules are tlie 

 lai'Kest anil are arranged in 

 filaments which cnnstitute the 

 skeletiin. A cross section of 

 one of these skeleton iilanients 

 will sliow the cnt-olf ends of 

 three or four spicules, witli a 

 cementing material of spongi- 

 olin. In some instances, at 

 least, tlie.se spicules are hollow. 



SPICULES l.)F p'RESH-\V.\TEl-t SPiixi.l.:. 



Usually, the skeleton spicules are slightly curved, 

 and more oi' less covered with spinous processes. 

 They ai'e arranged in filaments of three or four 

 spicules lying side by side and with their ends over- 

 lapping the ends of succeeding spicules, all held 

 together by spongi<jlin. The Hlaraeuts e.vtend from 

 the base of the sponge to the sui-face. 



The sk(deton filaments furnish the sui)]>ort for the 

 cells which constitute the body of tlie s]Hinge. These 

 cells ai'e ai'i'anged so that eatdt one constitutes a i)art 

 of the witU of a small chamber which communicates 

 with one of tlio canals whose external openings are 

 the excurrent ])oi-es. 



Th(! entire; sponge is covered with au epidei'iiiis, in 

 which are embedded di.'rmal spicides, usiudly smaller 

 than the skeleton s]3icules. 



