Further, as already mentioned, the eye-muscles in Ammocœtes must be considered by themselves; they do not belong in structure or position to the longitudinal somatic muscles innervated by the spinal nerves; their structure is not the same as that of the tubular constrictor or branchial muscles, but resembles that structure somewhat; their position is dorso-ventral rather than longitudinal; they may be looked upon as a primitive type of somatic muscles segmentally arranged, the direction of which was dorso-ventral.
Anderson also has shown that the time of medullation of the nerves supplying these muscles is much earlier than that of the nerves belonging to the somatic trunk-muscles, their medullation taking place at the same time as that of the motor nerves supplying the striated visceral muscles; and Sherrington has observed that these muscles do not possess muscle-spindles, while all somatic trunk-muscles do. Both these observations are strong confirmation of the view that the eye-muscles must be classified in a different category to the ordinary somatic trunk muscle group.
What, then, is the interpretation of these various embryological and anatomical facts?
Remembering the tripartite division of each segmental nerve-group in Limulus into (1) dorsal or sensory somatic nerve, (2) appendage-nerve, and (3) ventral somatic nerve, I venture to suggest that the three nerves—the oculomotorius, the trochlearis, and the abducens—represent the ventral somatic nerves of the prosoma, and partly also of the mesosoma; that they are nerves, therefore, which may have originally contained sensory fibres, and which still contain the sensory fibres of the eye-muscles themselves, as stated by Sherrington. According to this suggestion, the eye-muscles are the sole survivors of the segmental dorso-ventral somatic muscles, so characteristic of the group from which I imagine the vertebrates to have sprung. In the mesosomatic region the dorso-ventral muscles which were retained were those of the appendages and not of the mesosoma itself, because the presumed ancestor breathed after the fashion of the water-breathing Limulus, by means of the dorso-ventral muscles of its branchial appendages, and not after the fashion of the air-breathing scorpion, by means of the dorso-ventral muscles of the mesosoma. The only mesosomatic dorso-ventral muscles which were retained were those of the foremost mesosomatic segments, i.e. those supplied by the VIth nerve, which were preserved owing to their having taken on a prosomatic position and become utilized to assist in the movements of the lateral eyes.
Let us turn now to the consideration of the corresponding musculature in Limulus and in the scorpion group. These muscles constitute the markedly segmental muscles to which I have given the name 'dorso-ventral somatic muscles.' They are most markedly segmental in the mesosomatic region, both in Limulus and in Scorpio, each mesosomatic segment possessing a single pair of these vertical mesosomatic muscles, as Benham calls them (cf. Fig. [58] (Dv.)). In the prosomatic region the corresponding muscles are not so clearly defined in Limulus; they are apparently attached to the plastron forming the group of plastro-tergal muscles. From Benham's description it is sufficiently evident that they formed originally a single pair to each prosomatic segment.
In Scorpio, according to Miss Beck, the dorso-ventral prosomatic muscles are situated near the middle line on each side and form the following well-marked series of pairs of muscles, shown in Fig. [110], A, taken from her paper, and thus described by her:—
1. The dorso-cheliceral-sternal muscle (61) is the most anterior of the dorso-ventral muscles. It is very small, and is attached to the carapace near the median line anteriorly to the central eyes.
2. The median dorso-preoral-entosclerite muscle (62) is a large muscle, between which and its fellow of the opposite side the eyes are situated. It is attached dorsally to the carapace and ventrally to the pre-oral entosclerite.
3. The anterior dorso-plastron muscle (63) is attached dorsally to the carapace in the middle line, being joined to its fellow of the opposite side. They separate, and are attached ventrally to the plastron. Through the arch thus formed the alimentary canal and the dorsal vessel pass.
4. The median dorso-plastron muscle (64) is attached dorsally to the posterior part of the carapace. It runs forward on the anterior surface of the posterior flap of the plastron to the body of the plastron, to which it is attached.