274 THE ORIGIN OF VERTEBRATES 



cceloinic cavity in Limulus aud the muscles derived from the ventral 

 mesoderm, in all probability the muscles of the lower lip in the 

 lamprey (cf. Chap. IX.), which represents the metastoma; while the 

 muscles derived from the dorsal mesoderm, i.e. Miss Piatt'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, i.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 interned 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 external 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 



