DERMIC ORIGIN OF THE SENSORY ORGANS. 197 



This view is fully confirmed by the results of Comparative 

 Anatomy. Comparative Anatomy shows that many lower 

 animals possess no nervous system, although, in common 

 with higher animals, they exercise the functions of sensation, 

 volition, and thought. In the Primitive Animals (Protozoa), 

 v/iiich do not even form germ-layers, of course the nervous 

 system, like the skin-covering, is wanting. Even in the 

 second main division of the animal kingdom in the Metazoa 

 or Intestinal Animals there is at first no nervous system. 

 The functions of these are performed by the simple cell- 

 layer of the exoderm, which the lower Intestinal Animals 

 have inherited directly from the Gastraea (Fig. 209, e). This 

 is the case in the lowest Plant Animals (Zoophyta), the Gas- 

 trgeads, Sponges, and the lowest Hydroid Polyps, which are 

 but little higher than the Gastraeads. Just as all the vege- 

 tative functions of these are performed by the simple intes- 

 tinal layer, so all the animal functions are discharged by 

 the equally simple skin-layer. The simple cell stratum of 

 the exoderm is, in these, skin-covering, motive apparatus, 

 and nervous system simultaneously. 



Most probably the nervous system was also wanting in a 

 large proportion of those Primitive Worms (Archelminthes) 

 which were developed directly from the Gastrseads. Even 

 those Primitive Worms in which the two primary germ-layers 

 had already split into the four secondary germ-layers (Plate 

 V. Fig. 10), seem not to have possessed a nervous system 

 distinct from the skin. The skin-sensory layer must, even in 

 these long-extinct Worms, have been at once skin-covering 

 and nerve-system. But already in the Flat Worms (Platel- 

 minthes), and especially in the Gliding Worms (Turbellaria) 

 which of all existing forms approach nearest to the Primitive 



