Contents. 
Introduction : Pag. 
1, History 29 
a) The structure of the nerve-tubes 30 
b) The structure of the ganglion cells 32 
c) The structure of Leydig's dottecl substance 38 
d) The combination of the ganglion cells with each other 64 
Dr. B. Rawitz's paper on the central nerve-system of the Acephala. . . 65 
2, The material examined 72 
3, Methods of investigation 73 
Description of my investigations : 
1. The structure of the nerve-tubes in invertebrates 81 
Summary 97 
2. The structure of the ganglion cells, and their processes, in invertebrates . 98 
Summary 120 
3. The structure of Leydig's dotted substae 122 
Summary 144 
4. The combination of the ganglion cells with each other, and the function 
of the protoplasmic processes 145 
5. The nervous elements of Amphioxus and Myxine 149 
6. The combination of the nerve-tubes with each other 164 
7. The function of the ganglion cells 167 
List of the principal Literature 172 
Explanation of the plates 194 
