38 



ANIMAL STUDIES 



to assign such forms to a definite position among the 



sponges. 



36. Structure of a simple sponge. In the simpler sponges 



the body is usually vase-shaped (Fig. 16), with the base 



fastened to some foreign 

 object, while at an oppo- 

 site end an opening leads 

 into a comparatively large 

 internal cavity. This lat- 

 ter space is also put in 

 communication with the 

 exterior by a multitude of 

 minute pores which pene- 

 trate the body wall. In 



FIG. 15. Various forms of sponges, natural size. (From Nature.) 



the living condition currents of water continually pass 

 through these smaller canals, and out of the large termi- 

 nal opening, thus bringing within reach of the body minute 



