218 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 
Haeckel, into a neutral kingdom, owing to the absence 
of sexual reproduction, we must likewise agree with 
him in attributing to the Spongiadze 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 
along 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 
Ccelenterata, Polypes, and Medusz, 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 spongiadze may be 
