THE EVIDENCE OF THE RESPIRATORY APPARATUS 157 
These observations of van Wijhe lead directly to the following 
conclusion. In the cranial region there is evidence of a double set 
of segments, which may be called somatic and splanchnic. The 
somatic segments, consisting of the outer skin and the body muscu- 
lature, are doubly innervated as are those of the spinal cord by a 
series of ventral motor roots, the oculomotor or IIIrd nerve, the 
-trochlear or [Vth nerve, the abducens or VIth nerve, and the hypo- 
glossal or XIIth nerve, and by a series of dorsal sensory roots, the 
sensory part of the trigeminal or Vth nerve. But the splanchnic 
segments are innervated by single roots, the vagus or Xth nerve, 
glossopharyngeal or [Xth nerve, facial or VIIth nerve, and trigeminal 
or Vth nerve, which are mixed, containing both sensory and motor 
fibres, thus differing markedly from the arrangement of the spinal 
nerves. 
From this sketch it follows that the arrangement seen in the 
spinal cord, would result from the cranial arrangement if this third 
system of lateral roots were left out. Further, since the cranial 
system is the oldest, we must search in the invertebrate ancestor for 
a tripartite rather than a dual system of nerve-roots for each segment ; 
a system composed of a dorsal root supplying only the sensory nerves 
of the skin-surfaces, a lateral mixed root supplying the system con- 
nected with respiration with both sensory and motor fibres, and a 
ventral root supplying the motor nerves to the body-musculature. 
COMPARISON OF THE APPENDAGE NERVES OF LIMULUS AND BRANCHI- 
-PUS TO THE LATERAL Root SYSTEM OF THE VERTEBRATE. 
If the argument used so far is correct, and this tripartite system 
of nerve-roots, as seen in the cranial nerves of the vertebrate, really 
represents the original scheme of innervation in the paleostracan’ 
ancestor, then it follows that each segment of Limulus ought to be 
supplied by three nerves—(1), a sensory nerve supplying its own 
portion of the skin-surface of the prosomatic and mesosomatic 
carapaces; (2), a lateral mixed nerve supplying exclusively the 
appendage of the segment, for the appendages carry the respiratory 
organs; and (3), a motor nerve supplying the body-muscles of the 
segment. 
It is a striking fact that Milne-Edwards describes the nerve-roots 
in exactly this manner. The great characteristic of the nerve-roots 
