ON THE ORIGIN OP VEETEBRATES FROM ARACHNIDS. 323 



proximal ends of the primitive motor and sensory nerves 

 remain unfused, forming for each spinal nerve two distinct 

 branches and two roots. The abdominal sympathetic nerves 

 are very small, and their relation to nerves of the adult has 

 not been in all cases determined. 



The sensory root of the adult spinal nerve arises near 

 the neural surface of the neuromere. Besides the ordinary 

 fibres, it contains an axial bundle of coarse and deeply stain- 

 able nerve-tubes, surrounding which is an elongated mass of 

 small ganglion-cells (Fig. 2, sp. g.). 



The motor root arises near the haemal surface of the neu- 

 romere, and is distinguished by its light colour and by the 

 absence of the dark nerve-tubes and ganglion-cells. A short 

 distance from the neuromere the motor and sensory roots 

 unite to form a single nerve, which, on reaching the sides of 

 the body, divides into two branches, one extending backwards, 

 the other laterally (Fig. 4). 



The above features are not so clearly defined in the caudal 

 segments. 



Thus the abdominal spinal nerves of Scorpio resemble the 

 spinal nerves of Vertebrates — (1) In their origin from two or 

 more originally separate nerves; (2) in the failure of the distal 

 and proximal ends of the nerves to unite; (3) in the motor 

 and sensory roots arising respectively from the haemal and 

 neural surfaces of the nerve-cord ; (4) in the presence of two 

 kinds of nerve-tubes in the sensory root ; (5) in the presence 

 of a collection of ganglion-cells in the sensory root, between 

 the nerve-cord and the point where the two roots unite ; (6) in 

 the origin, as will be shown later, of this ganglion from a spe- 

 cialised part of a dark lateral border of the ventral cords, 

 comparable with the neural crest of Vertebrates. 



III. The Thoracic or Cranial Nerves of Scorpio remain 

 separate throughout life ; hence they diff'er from the abdominal 

 nerves in the same vray that it is supposed some of the Verte- 

 brate cranial nerves differ from the spinal ones. 



Examined more closely, we find that of the three pairs of 



