PROSOMATIC SEGMENTS OF LIMULUS 247 



As these appendages did not carry any vital organs such as 

 branchiae, hut were mainly locomotor and masticatory in function, it 

 follows that their disappearance as such would be much more com- 

 plete than that of the mesosomatic branchial appendages. Most 

 probably, then, in the higher vertebrates no trace of such appendages 

 might be left ; consequently the segmentation due to their presence 

 would be very obscure, so that in this region the very reverse of what 

 is found in the reoion of the vagus nerve would be the rule. There 

 branchiomeric segmentation is especially evident, owing to the per- 

 sistence of the branchial part of the branchial appendages; here, 

 owing to the disappearance of the appendages, the segmentation is 

 no longer branchiomeric, but essentially mesomeric in consequence 

 of the persistence of the somatic eye-muscles. 



In addition to the evidence of the appendages themselves, the 

 number of prosomatic segments is well marked out in all the 

 members of the scorpion group by the divisions of the central 

 nervous system into well-defined neuromeres in accordance with the 

 appendages, a segmentation the reminiscence of which may still 

 persist after the appendages themselves have dwindled or disappeared. 

 In accordance with this possibility we see that one of the most 

 recent discoveries in favour of a number of segments in the head- 

 region of the vertebrate is the discovery in the early embryo of a 

 number of partial divisions in the brain-mass, forming a system of 

 cephalic neuromeres which may well be the rudiments of the well- 

 defined cephalic neuromeres of animals such as the scorpion. 



The Evidence of the Peosomatic Musculatuke. 



Even if the appendages as such become obscure, yet their muscles 

 might remain and show evidence of their presence. The most per- 

 sistent of all the appendage-muscles are the basal muscles which pass 

 from coxa to carapace and are known by the name of tergo-coxal 

 muscles. They are large, well marked, segmentally arranged muscles, 

 dorso- ventral in direction, and, owing to their connecting the limb 

 with the carapace, are likely to be retained even if the appendage 

 dwindles away. 



The muscular system of Limulus and Scorpio has been investi- 

 gated by Benham and Miss Beck under Lankester's direction, and the 

 conclusions to which Lankester comes are these — 



