2l8 THE DOCTRINE OF DESCENT. 



expectation that at least here and there, in the indi- 

 vidual development of single representatives of the vari- 

 ous families, witnesses of their common derivation should 

 come to light. This likewise occurs, and to such a de- 

 gree that in the earliest larval stages a link is estab- 

 lished between the lowest and the highest animals. If 

 a number of groups of the lowest living beings, in which 

 the various vital functions of nutrition, irritability, mo- 

 tion, and reproduction are supplied by amorphous proto- 

 plasm, — if these be separated, as by Haeckel, into a 

 neutral kingdom, owing to the absence of sexual repro- 

 duction, we must likewise agree with him in attributing 

 to the Spongiadae ranking next to the Protista, the name 

 of animals, on account of their sexual propagation and 

 the nature of their embryonic development and first larval 

 phases. 



Haeckel has bestowed on one larval stage of the 

 calcareous sponges the title of Gastrula, wherein the 

 animal represents a sac, or, in other words, a stomach 

 provided with a mouth-like orifice. The walls are 

 formed of two rows of cells, the outer one consisting of 

 ciliated cells; that is to say, each cell is furnished with 

 a long filament. At the orifice of the sac, the outer row 

 merges into the inner one, and from these two mem- 

 branes the body of the sponge is constructed in a definite 

 manner. Now, if this Gastrula larva reappears in the 

 Coelenterata, Polypes, and Medusae, in which the gradual 

 development from the two membranes, the entoderm 

 and ectoderm, 'into the most complex forms has long 

 been known; and if, as Haeckel has further shown, the 

 osculum, or larger opening of the spongiadae may be 

 closely compared with the mouth of the polype and 



