In all this group of animals, the evidence as to the number of segments in either the prosomatic or mesosomatic regions is given by—
1. The number of appendages.
2. The segmental arrangement of the muscles of the prosoma or mesosoma respectively.
3. The segmental arrangement of the cœlomic or head-cavities.
4. The divisions of the central nervous system, or neuromeres, together with their outgoing segmental nerves.
It follows, therefore, that if from any cause the appendages are not apparent, as is the case in many fossil remains, or have dwindled away and become insignificant, we still have the muscular, cœlomic, and nervous arrangements left to us as evidence of segmentation in these animals, just as in vertebrates.
In this prosomatic region, we find in Limulus the same tripartite division of the nerves as in the mesosomatic region, so that the nerves to each segment may be classed as (1) appendage-nerve; (2) sensory or dorsal somatic nerve, supplying the prosomatic carapace; (3) motor or ventral somatic nerve, supplying the muscles of the prosoma, and containing possibly some sensory fibres. The main difference between these two regions in Limulus consists in the closer aggregation of the prosomatic nerves, corresponding to the concentration of the separate ganglia of origin in the prosomatic region of the brain.
The number of prosomatic segments in Limulus is not evident by examination of the prosomatic carapace, so that the most reliable guide to the segmentation of this region is given by the appendages, of which one pair corresponds to each prosomatic segment.
The number of such segments, according to present opinion, is seven, viz.:—
(1) The foremost segment, which bears the cheliceræ.