TYPE OF DEVELOPMENT OF ARTICULATA. 



S3 



rounded itself with a germinal membrane, a portion of it 

 thickens into a long germinal stria, resembling an elon- 

 gated ellipse. This is the rudiment of the ventral side of 

 the future animal. A groove then divides it into the two 

 germinal laminae, and transverse striae next make their 

 appearance, the indications of the so-called primordial 

 segments. The symmetrical disposition of the organs, 

 and the integration of the body out 

 of consecutive segments, is herewith 

 initiated. All further development 

 emanates from these primordial seg- 

 ments, which are the standard of the 

 Annelids or higher Vermes ; w^hile in 

 the Articulata, projections and ap- 

 pendages of these segments develop 

 into feelers, manducatory apparatus 

 and legs, and by their heterogeneous 

 integration in the regions of the 

 head, and of the middle and posterior 

 portions of the body, give rise to the 

 vast variety within the type. In each 

 particular case we see what is special 

 emanate from what is more homogeneous and undiffer- 

 entiated, and this is likewise corroborated by the more 

 advanced phase portrayed in the diagram (fig. 6). It 

 represents the embryo of the great black-beetle (Hydro- 

 philus piceus) on its ventral side. The antennae (f), the 

 three pair of oral appendages (;;/), and the three pair of 

 legs, are as yet little distinguished. In the further course 

 of development, the lateral portions grow tow^ards the 

 back, in the centre of which they finally meet. As 

 compared with the Vertebrata, it may hence be said 



F.'.G. 6. 



