ENDOSKELETAL SYSTEMS OF LIMULUS AND SCORPIO. 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—cow. signifies coxal insertion, deut. insertion 
into the deutomerite, and arthr. insertion into the arthrodial membrane between the 
adjacent coxz or the coxa and sternal sclerite. 
Limulus. Scorpio. 
ils > ai) aiaGl 3))l «4 a seer MomeonmICNeES none. 
Illy, BRAY BB Bye Eee etm Aas ee 72 (cox.), 75 (cox.), 76 (cox.). 
WE SOOO soe ee Me! Uae) 75 (deut.), 74 (cox.). 
JW gS S205) cee a 77 (deut.), 78 (cox.), 79 (arthr.). 
Wie CO eS ere a 80 (deut.), 81 (arthr.). 
Wil) 44, 40465 4 2 ee 82 (deut.), 83 (arthr.), 86 @ (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 Zimulus. 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, ¢. 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 :— 
