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 diam-am ^rives the section of a larva of this 

 description, which at this period is nothing more than a 

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







Fig. c. 



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 



