248 THE ORIGIN OF VERTEBRATES 



The simple musculature of the primitive animal from which hoth 

 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. 



3. 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 hoth Limulus and the Scorpion group ; so that we may 

 safely conclude that in the Pakeostraca 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 palaeostracan 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 

 Illrd, IVth, and YIth 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 



