In addition to the evidence of the appendages themselves, the number of prosomatic segments is well marked out in all the members of the scorpion group by the divisions of the central nervous system into well-defined neuromeres in accordance with the appendages, a segmentation the reminiscence of which may still persist after the appendages themselves have dwindled or disappeared. In accordance with this possibility we see that one of the most recent discoveries in favour of a number of segments in the head-region of the vertebrate is the discovery in the early embryo of a number of partial divisions in the brain-mass, forming a system of cephalic neuromeres which may well be the rudiments of the well-defined cephalic neuromeres of animals such as the scorpion.
The Evidence of the Prosomatic Musculature.
Even if the appendages as such become obscure, yet their muscles might remain and show evidence of their presence. The most persistent of all the appendage-muscles are the basal muscles which pass from coxa to carapace and are known by the name of tergo-coxal muscles. They are large, well marked, segmentally arranged muscles, dorso-ventral in direction, and, owing to their connecting the limb with the carapace, are likely to be retained even if the appendage dwindles away.
The muscular system of Limulus and Scorpio has been investigated by Benham and Miss Beck under Lankester's direction, and the conclusions to which Lankester comes are these—
The simple musculature of the primitive animal from which both Limulus and the scorpions arose consisted of—
1. A series of paired longitudinal dorsal muscles passing from tergite to tergite of each successive segment.
2. A similar series of paired longitudinal ventral muscles.
3. A pair of dorso-ventral muscles passing from tergite to sternite in each segment.
4. A set of dorso-ventral muscles moving the coxa of each limb in its socket.
5. A pair of veno-pericardial muscles in each segment.