THE SENSORY APPARATUS. 195 



think, securely founded on the Gastrula that most impor- 

 tant of all the germ-forms of the animal kingdom which 

 we find recurs in similar form in the germ-history of the 

 most different classes of animals. This significant germ- 

 foiTQ points unmistakably to a parent-form (the Gastrsea) 

 common to all animals, the Protozoa alone excepted ; in 

 this long extinct parent-form the entire body of the animal 

 consisted throughout life of the two primary germ-layers, as 

 is yet the case, for a short time, in the Gastrula. In the 

 Gastrsea the simple skin-layer did actually represent all the 

 animal organs and functions, and the simple intestinal layer, 

 on the other hand, all the vegetative organs and functions ; 

 potentially, this is even yet the case in the Gastrula. 



In studying the development of the first important 

 part of the animal sphere, the sensory apparatus, or sen- 

 sorium, we shall now find how well adapted this Gastrsea 

 Theory is to explain, not only in a morphological but 

 in a physiological sense, the most important facts in the 

 history of evolution. This sensory apparatus consists of two 

 very distinct parts, having, apparently, nothing in common : 

 in the first place, the external skin-covering (Derma), 

 together with its appendages, the hair, nails, sweat-glands, 

 etc. ; and, secondly, the nervous system, situated internally. 

 The latter includes the central nervous system (brain and 

 spinal chord), the peripheric brain-nerves and medullary 

 nerves, and finally, the organs of sense. In the fully 

 developed vertebrate body these two main constituents of 

 the sensorium are entirely separate ; the skin lying entirely 

 cxternallv on the body, while the central nervous system 

 is within, and quite separate from the former. The two 

 are connected merely by a portion of the peripheric nerve- 



