Fig. 104.—Eurypterus Fischeri. (From Holm.)
In the living scorpion and in Limulus the nerves to this pair of appendages undoubtedly arise from the foremost prosomatic ganglia, and the reason why they appear to belong to the supra-œsophageal brain-mass has been made clear by Brauer's investigations on the embryology of Scorpio; for he has shown that the cheliceral ganglia shift from the ventral to the dorsal side of the œsophagus during development, thus becoming pseudo-supra-œsophageal, though in reality belonging to the infra-œsophageal ganglia. This cheliceral pair of appendages is, in all probability, homologous with the second pair of antennæ in the crustacea.
I conclude, then, that the cheliceræ must truly be included in the prosomatic group, but that they stand in a somewhat different category to the rest of the prosomatic appendages, inasmuch as they take up a very median anterior and somewhat dorsal position, and their ganglia of origin are also exceptional in position.
Next for consideration come the chilaria (7 in Fig. [103]), which Lankester did not consider to belong to appendages at all, but to be a peculiar pair of sternites. Yet their very appearance, with their spinous hairs corresponding to those of the other gnathites and their separate nerve-supply, all point distinctly to their being a modified pair of appendages, and, indeed, the matter has been placed beyond doubt by the observations of Kishinouye, who has found embryologically that they arise in the same way as the rest of the prosomatic appendages, and belong to a distinct prosomatic segment, viz. the seventh segment. In accordance with this, Brauer has found that in the scorpion there is in the embryo a segment, whose appendages degenerate, which is situated between the segment bearing the last pair of thoracic appendages and the genital operculum—a segment, therefore, comparable in position to the chilarial segment of Limulus.
Coming now to the five locomotor appendages, we find that they resemble each other to a considerable extent in most cases, with, however, certain striking differences. Thus in Limulus they are chelate, with their basal joints formed as gnathites, except in the case of the fifth appendage, in which the extremity is modified for the purpose of digging in the sand. In Pterygotus, Slimonia, Eurypterus, the first four of these appendages are very similar, and are called by Huxley and Woodward endognaths; in all cases they possess a basal part or sterno-coxal process, which acts as a gnathite or foot-jaw, and a non-chelate tactile part, which possesses no prehensile power, and in most cases could have had no appreciable share in locomotion, called by Huxley and Woodward the palpus. These small palps were probably retractile, and capable of being withdrawn entirely under the hood. The fifth appendage is usually different, being a large swimming organ in Pterygotus, Eurypterus, and Slimonia (Figs. 8 and 104), and is known as the ectognath.
Finally, in Drepanopterus Bembycoides, as stated by Laurie, all five locomotor appendages are built up after the same fashion, the last one not being formed as a paddle-shaped organ or elongated as in Stylonurus, but all five possess no special locomotor or prehensile power. According to Laurie this is a specially primitive form of the group.
It is significant to notice from this sketch that with the absence of special prehensile terminations such as chelæ, or the absence of special locomotor functions such as walking or swimming, these appendages tend to dwindle and become insignificant, taking up the position of mere feelers round the mouth, and at the same time are concentrated and pressed closely together, so that their appendage-nerves must also be close together.
This sketch therefore shows us that—
Of the six foremost prosomatic appendages, the cheliceræ and the four endognaths were, at the time when the vertebrates first appeared, in very many cases dwindling away; the latter especially no longer functioned as locomotor appendages, but were becoming more and more mere palps or tentacles situated round the mouth, which could by no possibility afford any help to locomotion.
On the contrary, the sixth pair of appendages—the ectognaths—remained powerful, being modified in many cases into large oar-like limbs by which the animal propelled itself through the water.