376 THE ORIGIN OF VERTEBRATES 
so, also, is the nerve to the flabellum in Limulus, while the large size 
of the auditory nerve in 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- 
pteridz 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-organs 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[th nerve represents a 
segment immediately posterior to that of the VIIth, it is evident 
that an organ in the situation of the pecten, immediately posterior 
to the operculum, 7.¢. according to my view, posterior to the segment 
originally represented by the VIIth 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. 
