NATURE STUDY IN SCHOOLS. 217 



Now it is extremely probable that the original form of sponges was a 

 single cylinder, pushing up from a base, but as under certain conditions, for 

 example, where a sponge grew in a swift tideway, it became advantageous 

 for the species to produce a number of cylinders which adhered together, a 

 sponge of a more complicated structure was the result. 



As seen, although the tube sponge nearly always, or always, grows in 

 quiet waters it, exhibits an inclination to form masses of tubes, that is, it takes 

 one step in the direction of evolving a species with several tubes joined to- 

 gether. 



Fig. 99. 



■ 4 t "iff '.• 



Tube Sponge. 



Such a species we actually find in a specimen of sponge closely allied 

 to the tube sponge, in fact, belonging to the same genus ( Verongia ) . This 

 is the filamentous sponge, and in this species we not only find a number of 

 tubes joined together, but the tubes are greatly reduced in size. A specimen 

 of the filamentous sponge is figured on page 179. Here, as will be seen, 

 there are about eight tubes joined side by side. Thus step by step, we can 

 trace the evolution of the sponge through such forms as the net sponge, fig. 



