ELISHA MITCHELL SCIENTIFIC SOCIETY. 39 



P. dilopha has been derived from P. monolopha bv an 

 increase in the thickness of the mesoderm lying beneath 

 the surface of the sponge. The wide afferent canals of P. 

 mojiolopJia become transformed into the narrow afferent 

 canals of P. dilopha. 



Plakina trilopJia goes a step farther in the direction of 

 complexity than does P. dilopha. It has probably been 

 derived from the latter species by the appearance of sec- 

 ondary folds in the radial efferent tubes; by the trans- 

 formation of the basal cavity into a system of lacunae, 

 owino^ to the increase in the number of the connectino^ 

 Strands of tissue between the basal layer and the part of 

 the sponge containing the flagellated chambers; and by a 

 complication in the afferent canals in consequence of which 

 they do not open each by a single aperature, but by a num- 

 ber of small apertures the surface pores. 



Schulze's conclusion that these species all lie in one line 

 of descent — that is, that the second species has been de- 

 rived from the first, and the third from the second — receives 

 as much support from a study of the spicules as of the 

 canal system, but here reference will have to be made to 

 the paper. 



From comparative anatomy, then, we conclude the phy- 

 logeny of the sponges to be something as follows: The 

 Olynthus is the ancestor of the group. The outgrowth of 

 radial tubes gave rise to the Sycon type. The growth of 

 the mesoderm and development of new^ endodermic diverti- 

 cula, coupled with the metamorphosis of the radial tubes 

 into flagellated chambers, produced the Leucons. The 

 non-calcareons sponges have been derived from t\pes more 

 or less like the Leucons. And the conclusion with regrard 

 to the germ layers is that the efferent system is entirely 

 endodermic, and the afferent system entirely ectodermic. 



Emhryological Evidence. Let us see now how far the 

 known facts of development support the above conclusions. 



