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 @ 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 
Gf the pair of muscles (63) belongs to the segment of the sixth loco- 
motor prosomatic appendages), 7c. 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 celomic cavity of Limulus, 
and therefore represents, as far as the prosomatic appendages are 
concerned, the first prosomatic appendage-pair, or the chelicere; the 
appendage-muscles being the muscles of the chelicere, 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 
