The second part of the mandibular cavity represents the 4th {274}coelomic cavity in Limulus and 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 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, _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 position 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 _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 {275}service, and would form an internal and not an external group of eye-muscles.