cam., camerostome; pl., plastron.
The evidence of the Xiphosura and of the Hemiaspidæ conclusively shows, in Woodward's opinion, that the Merostomata are closely related to the Trilobita, and the Hemiaspidæ especially are supposed to be intermediate between the trilobites and the king-crabs. They are characterized, as also Belinurus and Prestwichia, by the absence of any prosomatic appendages, so that in these cases, as is seen in Fig. [12] (p. [30]), representing Bunodes lunula, found in the Eurypterus layer at Rootziküll, we have an animal somewhat resembling Limulus in which the prosomatic appendages have either dwindled away and are completely hidden by the prosomatic carapace, or became so soft as not to be preserved in the fossilized condition. The appearance of the prosomatic carapace is, to my mind, suggestive of the presence of such appendages, for it is marked out radially, as is seen in the figure, in a manner resembling somewhat the markings on the prosomatic carapace of Mygale or Phrynus; the latter markings, as already mentioned, are due to the aponeuroses between the tergo-coxal muscles of the prosomatic appendages which lie underneath and are attached to the carapace.
A very similar radial marking is shown by Woodward in his picture of Hemiaspis limuloides, reproduced in Fig. 109, found in the Lower Ludlow beds at Leintwardine. This species has yielded the most perfect specimens of the genus Hemiaspis, which is recognized as differing from Bunodes by the possession of a telson.
It is striking to find that similar indications of segments have been found on the dorsal surface of the head-region in many of the most ancient extinct fishes, as will be fully discussed later on.
Fig.109.—Hemiaspis limuloides. (From Woodward.)
gl., glabellum.
The Evidence of Cœlomic Cavities.
In the head-region of the vertebrate, morphologists depend largely upon the embryonic divisions of the mesoderm for the estimation of the number of segments, and, therefore, upon the number of cœlomic cavities in this region, the walls of which give origin to the striated muscles of the head, so that the question of the number of segments depends very largely upon the origin of the muscles from the walls of these head-cavities. It is therefore interesting to examine whether a similar criterion of segmentation holds good in such a segmented animal as Limulus, or in the members of the scorpion group, in which the number of segments are known definitely by the presence of the appendages. In Limulus we know, from the observations of Kishinouye, that a series of cœlomic cavities are formed embryologically in the various segments of the mesosoma and prosoma, in a manner exceedingly similar to their mode of formation in the head-region of the vertebrate, and he has shown that in the mesosoma a separate cœlomic cavity exists for each segment, so that just as the dorso-ventral somatic muscles are regularly segmentally arranged in this region, so are the cœlomic cavities, and we should be right in our estimation of the number of segments in this region by the consideration of the numerical correspondence of these cavities with the mesomatic appendages. Similarly, in the vertebrate, we find every reason to believe that a single, separate head-cavity corresponds to each of the branchial segments in the opisthotic region, and therefore we should estimate rightly the number of segments by the division of the mesoderm in this region.
In the prosomatic region of Limulus, the dorso-ventral muscles are not arranged with such absolute segmental regularity as in the mesosomatic region, and Kishinouye's observations show that the cœlomic cavities in this region do not correspond absolutely with the number of prosomatic appendages. His words are:—