158 THE ORIGIN OF VERTEBRATES 
in Limulus as in other arthropods is the large 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-muscles, 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- 
