(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 cheliceræ, the question has arisen whether their nerves belong to the infra-œsophageal group, or are in reality supra-œsophageal.
It is instructive to observe the nature and the anterior position of this pair of appendages in the allied sea-scorpions, especially in Pterygotus, where the only chelate organs are found in these long, antennæ-like cheliceræ. In Slimonia and in Stylonurus they are supposed by Woodward to be represented by the small non-chelate antennæ 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 of these forms, for they possess only five free prosomatic appendages instead of six, as in Limulus and in Pterygotus. Similarly, Woodward only allowed five appendages for Pterygotus, so that his restorations were throughout consistent. Schmidt, in Pterygotus osiliensis has shown that the true number was six, not five, as seen in his restoration given in Fig. [8], A (p. [27]).
Fig. 103.—Ventral Surface of Limulus. (Taken from Kishinouye.)
The gnathic bases of the appendages have been separated from those of the other side to show the promesosternite or endostoma (End.).
With respect to Eurypterus, Schmidt figures an exceedingly minute pair of antennæ between the coxal joints of the first pair of appendages, thus making six pairs of appendages. Gerhard Holm, however, in his recent beautiful preparations from Schmidt's specimens and others collected at Rootziküll, has proved most conclusively that the cheliceræ of Eurypterus were of the same kind as those of Limulus. I reproduce his figure (Fig. [104]) showing the small chelate cheliceræ (1) overhanging the mouth orifice, just as in Limulus or in Scorpio.
So, also, since Woodward's monograph, Laurie has discovered in Slimonia acuminata a small median pair of chelate appendages exactly corresponding to the cheliceræ of Limulus, or of Eurypterus, or of Scorpio. We may, therefore, take it for granted that such was also the case in Stylonurus, and that the foremost pair of prosomatic appendages in all these extinct sea-scorpions were in the same position and of the same character as the cheliceræ of the scorpions.
