Of these groups of muscles, any one of which would indicate the number of segments, Groups 1 and 2 do not extend into the prosomatic region, and Group 5 extends only as far as the heart extends in the case of both Limulus and the Scorpion group; so that we may safely conclude that in the Palæostraca the evidence of somatic segmentation in the prosomatic region would be given, as far as the musculature is concerned, by the dorso-ventral somatic muscles (Group 3), and of segmentation due to the appendages by the dorso-ventral appendage musculature (Group 4).
Therefore, if, as the evidence so far indicates, the vertebrate has arisen from a palæostracan stock, we should expect to find that the musculature of the somatic segments in the region of the trigeminal nerve did not resemble the segmental muscles of the spinal region, was not, therefore, the continuation of the longitudinal musculature of the body, but was dorso-ventral in position, and that the musculature of the splanchic segments resembled that of the vagus region, where, as pointed out in Chapter IV., the respiratory muscles arose from the dorso-ventral muscles of the mesosomatic appendages. This is, of course, exactly what is found for the muscles which move the lateral eyes of the vertebrate; these muscles, innervated by the IIIrd, IVth, and VIth nerves, afford one of the main evidences of segmentation in this region, are always grouped in line with the somatic muscles of spinal segments, and yet cannot be classed as longitudinal muscles. They are dorso-ventral in direction, and yet belong to the somatic system; they are exactly what one ought to find if they represent Group 3—the dorso-ventral body-muscles of the prosomatic segments of the invertebrate ancestor.
The interpretation of these muscles will be given immediately; at present I want to pass in review all the different kinds of evidence of segmentation in this region afforded by the examination of the invertebrate, whether living or fossil, so as to see what clues are left if the evidence of appendages fails us. I will take in the first instance the evidence of segmentation afforded by the presence of the musculature of Group 4, even when, as in the case of many fossils, no appendages have yet been found. In such animals as Mygale and Phrynus the prosomatic carapace is seen to be marked out into a series of elevations and depressions, and upon removing the carapace we see that these elevations correspond with and are due to the large tergo-coxal muscles of the appendages; so that if such carapace alone were found fossilized we could say with certainty: this animal possessed prosomatic appendages the number of which can be guessed with more or less certainty by these indications of segments on the carapace.
In those forms, then, which are only known to us in the fossil condition, in which no prosomatic appendages have been found, but which possess, more or less clearly, radial markings on the prosomatic carapace resembling those of Phrynus or Mygale, such radial markings may be interpreted as due to the presence of prosomatic appendages, which are either entirely concealed by the prosomatic carapace or dorsal head-plate, or were of such a nature as not to have been capable of fossilization.
The group of animals in question forms the great group of animals, chiefly extinct, classified by H. Woodward under the order of Merostomata. They are divided by him into the sub-order of Eurypteridæ, which includes—(1) Pterygotus, (2) Slimonia, (3) Stylonurus, (4) Eurypterus, (5) Adelophthalmus, (6) Bunodes, (7) Arthropleura, (8) Hemiaspis, (9) Exapinurus, (10) Pseudoniscus; and the sub-order Xiphosura, which includes—(1) Belinurus, (2) Prestwichia, (3) Limulus.
Fig. 107.—Phrynus Margine-Maculata.
Ce., median eyes; le., lateral eyes; glab., median plate over brain; Fo., fovea.
Fig. 108.—Phrynus sp. (?). Carapace removed.