ENDOSKELETAL SYSTEMS OF LIMULUS AND SCORPIO. 3158 
It will be sufficient to point out here, by way of introduction, that necessarily in 
Scorpio the muscles to the appendages of the mesosoma are almost entirely suppressed 
(those of the last four pairs of appendages, which have become lung-books, entirely), 
whilst, on the other hand, the same muscles are large and functionally important in 
Limulus. Again, in Scorpio the free articulation of the segments of the mesosoma 
and of the metasoma is retained, and accordingly the musculature connected with that 
articulation is developed. In Limulus, on the other hand, the segments of the meso- 
soma are ankylosed, and there are consequently no intersegmental muscles. One 
great joint, however, that between prosoma and mesosoma, is retained by Limulus; 
and accordingly. in connexion with that one joint, we find an enormous and specialized 
muscular development, differing from anything in Scorpio. 
The most remarkable agreements to which the reader’s attention is directed before- 
hand are in respect of (1) a large number of the muscles attached to the prosomatic 
entochondrite ; (2) certain of the muscles attached to the pectines of Scorpio and the 
first gill-bearing appendage of Zimulus and to the related small entochondrites in both 
cases ; (3) the muscles arising from the pericardium and inserted into the investment of 
the great veinous sac, which in the one case lies at the base of a gill-book and in the other 
case forms the investment of the in-sunken lung-book. This is a most important agree- 
ment, since in each case the muscle must have a very definite and peculiar action in 
determining the flow of blood from the respiratory sinus to the heart. ‘These muscles 
were described as “ brides transparentes” by A. Milne-Edwards, in his account of the 
vascular system of Limulus. By Newport they were seen in the Scorpion, and figured 
in his drawing, fig. 27, pl. xiv. of the ‘ Philosophical Transactions’ for 1843 ; but they 
are not described or referred to by him in any way, and their significance has never yet 
been pointed out. 
Lastly, the agreement in the origin and insertion of the great dorso-ventral vertical 
muscles of the mesosoma isa prominent one. In the fourth Chapter of the present 
memoir a further discussion of the agreements and differences of the muscular system 
in Scorpio and Limulus will be found. 
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