ENDOSKELETAL SYSTEMS OF LIMULUS AND SCOEPIO. 313 



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 Limulus 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-Edw T ards, in his account of the 

 vascular system of Limulus. By Newport they were seen in the Scorpion, and figured 

 in his drawing, fig. 27, pi. 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 is a prominent one. In the fourth Chapter of the present 

 memoir a further discussion of the agreements and differences of the muscular system 

 in Scojpio and Limulus will be found. 



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