2l8 THE DOCTRINE OF DESCENT. 



constantly approximate in their origin, and vindicate the 

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

 vidual development of single representatives of the 

 various families, witnesses of their common derivation 

 should come to light. This likewise occurs, and to 

 such a degree that in the earliest larval stages a 

 link is established 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, motion, and reproduction are supplied by 

 amorphous protoplasm, — if these be separated, as by 

 Hacckel, into a neutral kingdom, owing to the absence 

 of sexual reproduction, we must likevWse agree with 

 him in attributing to the SpongiadcX 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 otlicr w^ords, 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 spongiadas may be 



