ON THE ORIGIN OF VERTEBRATES FROM ARACHNIDS. 329 



character between the cranial and spinal nerves ; at the same 

 time they present remarkable features not found elsewhere. 



The neural nerves to the four vagus neuromeres fuse com- 

 pletely to form the large pectinal nerve ; but the neural ganglia 

 at the base of the nerves retain to a certain extent their inte- 

 grity, forming what I have called the ganglion nodosum or 

 ganglion laminatum (owing to the remarkable concentric 

 laminae composing its medullary core), the ganglion fusi- 

 forme, and the ganglion minus (Fig. 1). 



The haemal nerves to the first vagus neuromere form two 

 distinct pairs, as in the typical cranial segments {h. v^. and 

 h. v^.). In each of the succeeding neuromeres the haemal 

 nerves have united with each other, forming three nerves with 

 double roots ; the latter decrease in length from the first pair 

 to the third, passing gradually into a condition like that in 

 the abdominal haemal nerves. 



A short distance from the brain all five haemal nerves form 

 a compact bundle extending backwards, some of the nerves 

 passing through the haemal wall of the cartilaginous cranium 

 or sternum, others passing out of the neural canal. The second 

 and third double nerves {v~. and v^., Fig. 4), some distance 

 from the brain, fuse to form a single nerve supplying the first 

 and second lung-books and the ventral surface of the body ; 

 on its way to these organs it passes over the ventral surface of 

 the liver, to which it possibly gives branches. The anterior 

 hgemal nerve of the first vagus neuromere [v^.) runs close to the 

 coxal gland, and, dividing into numerous branches, is lost on 

 the surface of a thick peritoneum-like membrane. The poste- 

 rior nerve (v^,) extends along the arthrodeal membrane supply- 

 ing numerous sense-organs in the skin of the sides and back 

 of the abdomen. The fourth vagus [v^.) supplies the skin 

 and longitudinal muscles on the ventral surface of the 

 abdomen. 



A small nerve arises from the ventral surface of the accessory 

 brain, and supplies the distal portion of the sexual ducts 

 (Figs. 1 and 4, x.). I could find no way of ascertaining 

 to what neuromere this nerve belongs. 



