272 THE ORIGIN OF VERTEBRATES 



muscles is shown just posteriorly to the brain-mass. This muscle 

 I will call the oblique muscle. 



Finally we come to the muscles (64) and (65), the median and 

 posterior dorso-plastron muscles, which run close together. Both 

 muscles are attached to the plastron, and, therefore, to that extent 

 belong to the prosomatic region ; they are attached dorsally close to 

 the junction of the prosoma and mesosoma. This position of the 

 first mesosomatic dorso-ventral muscle belonging to the opercular 

 segment may be compared with the position of the first mesosomatic 

 dorso-ventral muscle in Limulus which has become attached to the 

 prosomatic carapace ; in both cases we see an indication that the 

 foremost pair of mesosomatic dorso-ventral somatic muscles tend to 

 take up a prosomatic position. 



As to the pair of small muscles (64), I believe that they repre- 

 sent the dorso-ventral muscles of the seventh prosomatic segment 

 (if the pair of muscles (63) belongs to the segment of the sixth loco- 

 motor prosomatic appendages), i.e. they belong to the chilarial 

 segment or metastoma. 



I desire to draw especial attention to the fact that the dorso- 

 ventral muscle (64), which represents the seventh segment, always 

 runs close alongside the dorso-ventral muscle (65), which represents 

 the first mesosomatic or opercular segment. 



The comparison, then, of these two sets of facts leads to the 

 following conclusions : — 



The foremost prosomatic or trigeminal segment stood separate 

 and apart, being situated most anteriorly ; the musculature of this 

 segment does not develop, so that the only evidence of its presence 

 is given by the anterior ccelomic cavity. This corresponds, according 

 to my scheme, with the first or anterior coelomic cavity of Limulus, 

 and therefore represents, as far as the prosomatic appendages are 

 concerned, the first prosomatic appendage-pair, or the chelicene ; the 

 appendage-muscles being the muscles of the chelicerse, and the 

 dorso-ventral somatic muscles the pair of dorso-cheliceral sternal 

 muscles (61) in the scorpion. Both these sets of muscles, therefore, 

 dwindle and disappear in the vertebrate. 



Then came four segments fused together to form the preman- 

 dibular segment, the characteristic of which is the apparent non- 

 formation of any permanent musculature from the ventral mesoderm- 

 segments, and the formation of the eye -muscles innervated by the 



