24 Mr. H. M. Bemarrl nn 



the cephalothoracic segments, and later specialized for mus- 

 cular attachments, let us for a moment look at the analogous 

 structure in Limulus. Lankester * expressly tells us that 

 " the inter-entapophysial ligaments which run on each side, 

 right and left, along the dorsal surface of Limulus^ passing 

 from one entapophysis to the next, are of similar nature and 

 origin to the entochondrites," {. e. to the endosternite. Further, 

 we learn from Benham's researches f that these ligaments run 

 on the outer sides of the entapophyses. These two facts, it 

 seems to me, confirm the explanation of the endosternite of 

 Apus and Limulus as derivations of the longitudinal muscle- 

 bands given by me \. A comparison between cross sections 

 of Apus and Limulus leaves little doubt that these entapo- 

 physial ligaments are the structurally degraded homologues 

 of the dorsal muscle-bands, which, as in Apus^ once ran 

 outside the dorsal attachments of the dorso-ventral muscles. 

 In Limulus these muscles have lost their physiological signi- 

 ficance owing to fusion of the segments, and have become 

 ligamentous. 



If these ligaments, then, are derivatives of the dorsal longi- 

 tudinal muscle-bands, which seems to me in the highest 

 degree probable §, then the endosternite, which, according to 

 Lankester, is of exactly the same nature as these ligaments, 

 must be a derivative of the ventral muscle-bands, as I have 

 already maintained. 



The endosternite of Limulus thus viewed must be homo- 

 logous with that of Apus and not with that of the Arachnids, 

 which had, as we have seen, an entirely different origin. 

 Indeed the evidence which we have here obtained from this 

 short comparative study of the endosternal system appears 

 to be conclusive against the existence of any direct genetic 

 relationship between Limulus and Arachnids. 



The claims of relationship between the Trilobitcs, Z,/w?iZi/.<, 



* Cf. anted, p. 20. 



t "Muscular and Endoskeletal System of Z/'w«/»/.«," Trans. Zool. Soc. 

 vol. xi. (1885). 



X ' The Apodidop; Nature Series (189-2). 



§ I have carefully compared the musculntine oi Aj)iis vith IJenhams 

 detailed description nud drawings of tliat of Z//;i"/i^-'. and tind that that of 

 Limulus can be deduced from that of Ajuts (as I have before s-upjrested) 

 without dilliculty. This derivation further avoids the necessitv of 

 assuuiiu{r that any important muscles or whole series of muscles oitlier of 

 Scorpio or Limulus are new formations, which Lankester shows mnst 

 have been tlie case if Limulus and Scorpio are nearly related. I mitrht 

 here also mention that histolofjieally the muscle-tibres of Arachnids are 

 totally distinct from those of Crustacea. Limulus has, it appears. Crus- 

 tacean, or at least certainly not Arachnidan, muscle-tibres. 



