248 THE ORIGIN OF VERTEBRATES 
The simple musculature of the primitive animal from which both 
Limulus and the scorpions arose consisted of— 
1. A series of paired longitudinal dorsal muscles passing from 
tergite to tergite of each successive segment. 
2. A similar series of paired longitudinal ventral muscles. 
. A pair of dorso-ventral muscles passing from tergite to 
sternite in each segment. 
4. A set of dorso-ventral muscles moving the coxa of each limb 
in its socket, 
5, A pair of veno-pericardial muscles in each segment. 
Of these groups of muscles, any one of which would indicate the 
number of segments, Groups 1 and 2 do not extend into the proso- 
matic region, and Group 5 extends only as far as the heart extends 
in the case of both Limulus and the Scorpion group; so that we may 
safely conclude that in the Paleostraca the evidence of somatic 
segmentation in the prosomatic region would be given, as far as the 
musculature is concerned, by the dorso-ventral somatic muscles 
(Group 3), and of segmentation due to the appendages by the 
dorso-ventral appendage musculature (Group 4). 
Therefore, if, as the evidence so far indicates, the vertebrate has 
arisen from a paleostracan stock, we should expect to find that the 
musculature of the somatic segments in the region of the trigeminal. 
nerve did not resemble the segmental muscles of the spinal region, 
was not, therefore, the continuation of the longitudinal musculature 
of the body, but was dorso-ventral in position, and that the muscula- 
ture of the splanchic segments resembled that of the vagus region, 
_where, as pointed out in Chapter IV., the respiratory muscles arose 
from the dorso-ventral muscles of the mesosomatic appendages. 
This is, of course, exactly what is found for the muscles which move 
the lateral eyes of the vertebrate ; these muscles, innervated by the 
IlIrd, I1Vth, and VIth nerves, afford one of the main evidences of 
segmentation in this region, are always grouped in line with the 
somatic muscles of spinal segments, and yet cannot be classed as 
longitudinal muscles. They are dorso-ventral in direction, and yet 
belong to the somatic system; they are exactly what one ought to 
find if they represent Group 3—the dorso-ventral body-muscles of 
the prosomatic segments of the invertebrate ancestor. 
The interpretation of these muscles will be given immediately ; 
at present I want to pass in review all the different kinds of evidence 
eo 
