MORPHOLOGY OP BEAIN AND SENSE ORGANS OP LIMULUS. 87 



are more or less united. This is especially true of the Scorpion, 

 where I have shown that the abdominal nerves are built 

 on the type of true spinal nerves, for their proximal ends 

 remain separate to form two roots, the dorsal or neural one 

 being ganglionated, the haemal one being non-ganglionated. 



The accessory brain forms the fourth and last brain 

 region of Vertebrates and Arachnids. It is formed of from 

 two to four neuromeres derived from the trunk and added at 

 a comparatively late period to the head. In spite of their late 

 origin, they form the most specialized and completely fused 

 neuromeres of the head. In both cases almost every trace 

 of the metameres to which they belong has disappeared, 

 consequently their nerves have wandered backward to new 

 territory. The nerves to these four neuromeres in Scorpions 

 are very unlike all others in front or back of them, for the 

 four ganglionated neural nerves are fused into one group, and 

 the eight haemal ones into another. They are, on the other 

 hand, strikingly like the vagus nerves of Vertebrates in their 

 direction, distribution, and general appearance (see " Origin of 

 Vertebrates from Arachnids,^' fig. 1). In Limulus there are 

 only one or at most two neuromeres in this region, and there- 

 fore it has not such striking vagus characters as in Scorpions. 



The presence of these vagus segments is not confined to 

 Scorpions and Limulus, but is of very wide occurrence in 

 Arthropods. The last thoracic and first two or three abdominal 

 segments in Insects, and the four abdominal appendages bearing 

 segments in Spiders, probably belong here. Their presence in 

 trilobites is shown by a well-marked '' cervical suture " like 

 that which in the adult Limulus marks the presence of these 

 segments. 



After we have torn oflF the deceptive Arthropod mask that 

 disguises Limulus we discover that the nervous system, with 

 all its complex and intricate modifications, shows as a whole a 

 profound structural similarity to that of Vertebrates. This 

 similarity extends to important aggregations of parts, as well 

 as to many minute details, all of which could not on any 

 reasonable assumption have occurred accidentally or be due to 



