EARLIEST CONDITION OF LARVA. 51 



to the later and actual typical impress. With this object, 

 let us consider the larva of a calcareous sponge at the 

 stage which Haeckel has designated as the Gastrula 



phase. 



The diagram 



the section of a larva of this 



description, which at this period is nothing more than a 

 stomach provided with an orifice (fig. 5 o) ; its wall con- 



sists of two strata, or layers of cells. The cells of the 

 external stratum are distinguished from those of the 

 inner one by their elongated form, and the possession of 

 filaments serving as organs of locomotion. All subse- 

 quent development and differentiation, certainly not 

 very important in the sponges, may be traced to modi- 

 fications of these two membranes ; the external mem- 

 5 



