PROSOMATIC SEGMENTS OF LIMULUS 235 



away and become insignificant, we still have the muscular, ccelomic, 

 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 cara- 

 pace ; (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 concentra- 

 tion 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 chelicerae. 

 (2, 3, 4, 5, 6) The next five segments, which carry the paired 

 locomotor appendages ; and 



(7) The last segment, to which belongs a small abortive pair of 

 appendages, known by the name of the chilaria, situated between the 

 last pair of locomotor appendages and the operculum or first pair of 

 mesosomatic appendages. These appendages are numbered from 1-7 

 in the accompanying drawing (Fig. 103). 



Of these seven pairs of appendages, the significance of the first 

 and the last has been matter of dispute. With respect to the first 

 pair, or the chelicerae, the question has arisen whether their nerves 

 belong to the infra-oesophageal group, or are in reality supra- 

 cesophageal. 



It is instructive to observe the nature and the anterior position of 

 this pair of appendages in the allied sea-scorpions, especially in Ptery- 

 gotus, where the only chelate organs are found in these long, antennae- 

 like chelicerae. In Slimonia and in Stylonurus they are supposed by 

 Woodward to be represented by the small non-chelate antennas seen 

 in Fig. 8, B and C (p. 27), taken from Woodward. If such is the case, 

 then these figures show that a pair of appendages is missing in each 



