ENDOSKELETAL SYSTEMS OE LIMULUS AND SCOKPIO, 371 



the first pair of prosomatic limbs, except those above mentioned as arising from the 

 plastron (30 & 31). 



In the case of the following five pairs of appendages, the muscles attached to them 

 and to the plastron may be exhibited for comparison in a tabular form in the two 

 animals : in the series referring to Limulus all are attached to some part of the large 

 coxa ; in the series referring to Scorpio— cox. signifies coxal insertion, deut. insertion 

 into the deutomerite, and arthr. insertion into the arthrodial membrane between the 

 adjacent coxae or the coxa and sternal sclerite. 



Limulus. Scorpio. 



I. 30 and 31 none. 



II. 32, 33, 34 72 (cox.), 75 (cox.), 76 (cox.). 



III. 35, 36, 37 73 (deut.), 74 (cox.). 



IV. 38, 39, 40 77 (deut.), 78 (cox.), 79 (arthr.). 



V. 41, 42, 43 80 (deut.), 81 (arthr.). 



VI. 44, 45, 46, 47 82 (deut.), 83 (arthr.), 86 a (arthr.). 



In carrying out the comparison of the muscular relations of the plastron in Scorpio 

 and Limulus, it is necessary, in the next place, to point out that in Scorpio, a muscle 

 (85) passes from the hinder border of the body of the plastron into the genital oper- 

 culum. No such muscle occurs in Limulus. But a similar muscle (the first of the 

 series numbered 48 in Mr. Benham's description) passes from the second entochondrite 

 of Limulus, or entochondrite of the genital segment, into the genital operculum. Now, 

 in Scorpio there is no separate entochondrite in the genital segment, although there is a 

 separate entochondrite in the next following (the pectinigerous segment) corresponding to 

 the second mesosomatic entochondrite of Limulus ; and this entochondrite of the second 

 mesosomatic segment in Scorpio gives origin to muscles which descend into the pecten 

 (90, 91, 92), and correspond to the muscle 48 in the second mesosomatic segment of 

 Limulus as described by Mr. Benham. Hence it seems extremely probable that the 

 difference between Limulus and Scorpio as to the muscle and entochondrite of the 

 genital segment is to be accounted for by the fact that in Scorpio the entochondrite of 

 that (the first mesosomatic) segment has fused with the great prosomatic entochondrite 

 or plastron. This is in accordance with other indications of a tendency to draw up 

 structures to the prosoma, noticeable in the Scorpion, e. g. nerves. Accordingly the 

 muscle 85 of Scorpio (the operculo-plastral) is the equivalent of the internal branchial 

 (48) of the genital segment of Limulus. 



The longitudinal muscles attached to the plastron in Limulus and Scorpio belong 

 necessarily (when we exclude the adventitious or secondary muscles descending obliquely 

 to it from the tergites of a posterior region) to the typical ventral series. They may 

 be compared as follows : — 



