l>58 



NATURE STUDY IN SCHOOLS. 



across the top, but this may have been due to the fact that this individual 

 had a hole broken at the base as shown m the figure at b. On the other 

 hand, a young sponge (see fig. 126) does not have any extention of the 

 membrane at all. Without being able to speak definitely es to the fur.c- 

 tion of this membrane^ I am inclined to think that it is for the purpose of 

 retaining the ova within the tube for a certain period. 



SlLICJOUS SrONGES. 



In this group we find sponges in which the horny fiber is partly or 

 wholly replaced by spicules of silica, but as has been already stated, the 

 horny and spiculigenous so completely intergrade that it is impossible to 



Fig. 125. 



rtf 



Top of Tube Sponge. 



draw a hard and fast line between the two groups. The spicules of sponges 

 are of various forms, but one of the most common is needle shaped. ( See 

 tig. 127, B. ) The green encrusting sponge, figured on plate VIII, has spher- 

 ical spicules which are provided with points. These spicules remind one 

 strongly of the pollen grain of some plants. 



Fresh Water Spokges. 



These are often green in color and cling to bridge piers, submerged sticks, 

 weeds etc. Some species attain a large size, often growing a foot or more 

 in diameter. These are flat in form, but as a rule the green branching spe- 

 cies appear to produce ova in the fall and then the sponge dies, leaving the 

 e 6g s enclosed among the spicules. These ova are large enough to be seen 



