The Origin of Vertebrates

by Walter Holbrook Gaskell

The striking peculiarity of this muscle-pair, which distinguishes it from all other muscles in the scorpion, is the common attachment of the muscles of the two sides in the mid-dorsal line, so that the pair of muscles forms an arch through which the alimentary canal and dorsal blood-vessel pass.

The same dorso-ventral muscles are present in Phrynus, and in this animal the fibres of this pair of muscles (63) actually interlace before the attachment to the prosomatic carapace, so that the attachment of the muscle on each side overpasses the mid-dorsal line, and a true crossing occurs. In Fig. [108] the position of this pair of 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 a prosomatic position.

As to the pair of small muscles (64), I believe that they represent the dorso-ventral muscles of the seventh prosomatic segment (if the pair of muscles (63) belongs to the segment of the sixth locomotor prosomatic appendages), i.e. 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 cœlomic cavity. This corresponds, according to my scheme, with the first or anterior cœlomic cavity of Limulus, and therefore represents, as far as the prosomatic appendages are concerned, the first prosomatic appendage-pair, or the cheliceræ; the appendage-muscles being the muscles of the cheliceræ, 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 premandibular 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 oculomotor nerve from the dorsal mesoderm segments. These four segments have been so fused together that van Wijhe looked upon them as a single segment, and the premandibular cavity as the cavity of a single segment. They represent, according to my scheme, the segments belonging to the endognaths, i.e. the second, third, fourth, fifth pairs of prosomatic appendages; the premandibular cavity, therefore, represents the second cœlomic cavity in Limulus, which, according to Kishinouye, is the sole representative of the cœlomic cavities of the second, third, fourth, fifth prosomatic segments. The muscles derived from the ventral mesoderm-segments represent the muscles of these appendages, which therefore dwindle and disappear in the vertebrate, with the possible exception of the muscles innervated by the descending root of the trigeminal. The muscles derived from the dorsal mesoderm-segments, i.e. the eye-muscles supplied by the oculomotor nerve, represent the dorso-ventral somatic muscles of these four segments, muscles which are represented in the scorpion by the recti group of muscles, i.e. the median dorso-preoral-entosclerite muscles (62).

Then came two segments, the mandibular, in which muscles are formed both from the ventral and from the dorsal mesoderm-segments. From the former arose the main mass of muscles innervated by the motor root of the trigeminal, from the latter the superior oblique muscle and the mandibular muscle of Miss Platt, of which the former alone survives in the adult condition. These two segments are looked upon as a single segment by van Wijhe, of which the mandibular cavity is the cœlomic cavity. They represent, according to my scheme, the segments belonging to the sixth pair of prosomatic appendages or ectognaths, and the seventh pair, i.e. the chilaria or metastoma.

The first part, then, of the mandibular cavity represents the third cœlomic cavity in Limulus and the muscles derived from the ventral mesoderm, in all probability the muscles of the tongue in the lamprey (cf. Chap. IX.), which represents the ectognaths or sixth pair of prosomatic appendages, while the muscles derived from the dorsal mesoderm, i.e. the superior oblique muscles, represent the dorso-ventral somatic muscles of this segment, muscles which are represented in the scorpion group by the pair of anterior dorso-plastron or oblique muscles (63).