376 THE ORIGIN OF VERTEBRATES 



so, also, is the nerve to the flabellum in Limulus, while the large size 

 of the auditory nerve iu the vertebrate, in distinction to the size of 

 the auditory apparatus, has always aroused the attention of anatomists. 



Throughout this book my attention has been especially directed 

 to both Limulus and the scorpion group in endeavouring to picture to 

 myself the ancestor of the earliest vertebrates, because the Eury- 

 pteridse possessed such marked scorpion-like characteristics ; so that in 

 considering the origin of a special sense-organ, such as the vertebrate 

 auditory organ near the junction of the prosoma and mesosoma, it 

 seems to me that the presence of such marked special sense-orgaus as 

 the flabellum on the one hand and the pecten on the other, must 

 both be taken into account, even although the former is an adjunct 

 to a prosomatic appendage, while the latter represents, according to 

 present ideas, the whole of a mesosomatic appendage. 



From the point of view that the VII Ith nerve represents a 

 segment immediately posterior to that of the Vllth, it is evident 

 that an organ in the situation of the pecten, immediately posterior 

 to the operculum, i.e. according to my view, posterior to the segment 

 originally represented by the Vllth nerve, is more correctly situated 

 than an organ like the flabellum, which belongs to a segment anterior 

 to the operculum. 



On the other hand, from the point of view of the relationship 

 between the scorpions and the king-crabs, it is a possibly debatable 

 question whether the pecten really belongs to a segment posterior to 

 the operculum. The position of any nerve in a series depends upon its 

 position of origin in the central nervous system, rather than upon the 

 position of its peripheral organ. Now, Patten gives two figures of the 

 brain of the scorpion built up from serial sections. In both he shows 

 that the main portion of the pectinal nerve arises from a swelling, to 

 which he gives the name ganglion nodosum. This swelling arises on 

 each side in close connection with the origin of the most posterior 

 prosomatic appendage-nerve, according to his drawings, and posteriorly 

 to such origin he figures a small nerve which he says supplies the 

 distal parts of the sexual organs. This nerve is the only nerve which 

 can be called the opercular nerve, and apparently arises posteriorly 

 to the main part of the pectinal nerve. If this is so, it would 

 indicate that the pectens arose from sense-organs which were origi- 

 nally, like the flabella, pre-opercular in position, but have shifted to 

 a post-opercular position. 



