70 LOWER INVERTEBRATES. 



and showed that the polyps were but commensal parasites, having nothing to do 

 with the formation of the long stem of silicious threads wliich resembles a plume 

 of spun glass. 



Fiti. GO. — UoUenia carptuteria. 



This genus may be expected in depths varying from forty to one hundred fathoms 

 in northern seas, and in deeper water as we go towards the tropics, apparently requir- 

 ing an average temperature below 40° F. The sponge itself in the natural state, is 

 not as attractive as EiqylecieUa, being of a light-brown color, and friable when dry. 

 The top is usually occupied with a number of cloacal apertures surrounding a central 

 ])rominence which is in reality the end of the stem. The stem is spun by the tissues 



