274 THE ORIGIN OF VERTEBRATES 
ccelomic cavity in Limulus and the muscles derived from the ventral 
mesoderm, in all probability the muscles of the lower lip in the 
lamprey (¢f. Chap. IX.), which represents the metastoma; while the 
muscles derived from the dorsal mesoderm, i.c. Miss Platt’s pair of 
mandibular muscles, represent the dorso-ventral somatic muscles of 
this segment, muscles which are represented in the scorpion group 
by the pair of median dorso-plastron muscles (64). 
In connection with this last pair of muscles we find that the 
external rectus in the vertebrate represents the first dorso-ventral 
mesosomatic muscle in the scorpion, z.e. the posterior dorso-plastron 
muscle (65), and, as already mentioned (p. 267), that it always lies 
closely alongside the mandibular muscle, just as in the scorpion group 
muscle (65) always lies alongside muscle (64). 
In the invertebrate as well as in the vertebrate this muscle is a 
mesosomatic muscle which has taken up a prosomatic position. 
The question naturally arises, what explanation can be given of 
the fact that these dorso-ventral muscles attached on each side 
of the mid-dorsal line to the prosomatic carapace became converted 
into the muscles moving the eyeballs of the two lateral eyes? An 
explanation which must take into account not only the isolated posi- 
tion of the abducens nerve, but also the extraordinary course of the 
trochlearis. The natural and straightforward answer to this question 
appears to me quite satisfactory, and I therefore venture to commend 
it to my readers. 
I have argued the case out to myself as follows: The lateral eyes, 
must have been originally situated externally to the group of muscles 
innervated by the oculomotor nerve, for a sheet of muscle representing 
the superior internal and inferior rectus muscles could only wrap. 
round the internal surface of each lateral eye; i.e. the arrangement 
of the muscle-sheet, as in the scorpion, about two median eyes, is in 
the wrong position, for if those two eyes, which are the main eyes in 
the scorpion, were to move outwards to become two lateral eyes, then 
such a muscle-group would form a superior eternal and inferior rectus 
group. The evidence, however, of Eurypterus and similar forms is 
to the effect that the lateral eyes became big and the median eyes 
insignificant and degenerate. If, then, with the degeneration of the 
one and the increasing importance of the other, these lateral eyes 
came near the middle line, then the muscular group (62), which I 
have called the recti group, would naturally be pressed into their 
