158 THE ORIGIN OF VERTEBRATES 



in Limulus as in other arthropods is the largo appendage-nerve, 

 which is always a mixed nerve; in addition, there is a system of 

 sensory nerves to the prosomatic and mesosomatic carapaces, called 

 by him the epimeral nerves, which are purely sensory, and a third 

 set of roots which are motor to the body-inuseles, and possibly also 

 sensory to the ventral surface between the appendages. 



Moreover, just as in the vertebrate central nervous system the 

 centres of origin of the motor nerves of the branchial segmentation 

 are distinct from those of the somatic segmentation, so we find, from 

 the researches of Hardy, that a similar well-marked separation exists 

 between the centres of origin of the motor nerves of the appendages 

 and those of the somatic muscles in the central nervous system of 

 Branchipus and Astacus. 



In the first place, he points out that the nervous system of 

 Branchipus is of a very primitive arthropod type ; that it is, in fact, 

 as good an example of an ancient type as we are likely to find in the 

 present day ; a matter of some importance in connection with my 

 argument, since the arthropod ancestor of the vertebrate, such as I 

 am deducing from the study of Ammoccetes, must undoubtedly have 

 been of an ancient type, more nearly connected with the strange 

 forms of the trilobite era than with the crabs and spiders of the 

 present day. 



His conclusions with respect to Branchipus may be tabulated as 

 follows : — 



1. Each ganglion of the ventral chain is formed mainly for the 

 innervation of the appendages. 



2. Each ganglion is divided into an anterior and posterior division, 

 which are connected respectively with the motor and sensory nerves 

 of the appendages. 



3. The motor nerves of the appendages arise as well-defined axis- 

 cylinder processes of nerve-cells, which are arranged in well-defined 

 groups in the anterior division of the ganglion. 



4. A separate innervation exists for the muscles and sensory 

 surfaces of the trunk. The trunk-muscles consist of long bundles, 

 from which slips pass off to the skin in each segment; they are thus 

 imperfectly segmented. In accordance with this, a diffuse system 

 of nerve-fibres passes to them from certain cells on the dorsal surface 

 of each lateral half of the ganglion. These cell-groups are therefore 

 very distinct from those which give origin to the motor appendage- 



