70 



LOWER INVERTEBRATES. 



and showecT tliat the polyps were but commensal parasites, having nothing to do 

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

 of spun glass. 



Fig. 60. — Soltenia carpenteria. 



This genus mriy 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 Euplectella, being of a light-brown color, and friable when dry. 

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

 prominence which is in reality the end of the stem. The stem is spun by the tissues 



