An Ascon Sponge. 7 



through numerous pores and leaves through the osculum. 

 The pores are easily determined, in the preparation under 

 consideration, as small round openings free from nuclei and 

 surrounded by the rays of the spicules. 



Outside the endoderm (below in the present specimen), 

 and surrounding the spicules, are the mesoderm cells. The 

 mesoderm cells are not arranged as a single layer, but occur 

 irregularly through the mass of spicules. Their nuclei are 

 often seen as granular bodies along the rays of the spicules, 

 and their protoplasm appears as a faintly stained reticulum. 

 While the endoderm cells have a definite outline, the meso- 

 derm cells are irregular and multipolar. The spicular skele- 

 ton is formed by the activity of the mesoderm cells. 



Still a third layer of cells, the ectoderm, has been described. 

 Its cells form a very thin tissue, easily destroyed, and too 

 difficult of demonstration to be here considered. It covers 

 the outer surface of the animal, and is probably somewhat 

 soiled with foreign matter. 



Make a drawing illustrating the points above mentioned. 



Reproduction. Many of the larger specimens prepared 

 and mounted as above directed will be found to bear reproduc- 

 tive cells. These cells appear as large, deeply stained bodies 

 resting in the mesoderm and covered by the endoderm. 

 They are of two kinds, the male and the female. The male 

 cells or spermatozoa occur in spheres, each sphere containing 

 several hundred sperm-cells. The nuclei of the sperm-cells 

 give to the surface of the sphere a peculiar dotted appear- 

 ance, and are considerably smaller than the entodermal 

 nuclei. The sperm-spheres, however, are large, their diame- 

 ter being about one-third the length of a long ray of a 

 quadriradiate spicule. 



