524 



HISTORY OF THE HUMAN BODY 



in the other a definite vessel. This relationship suggests a cer- 

 tain homology between the appendages of Limulusand the gill- 

 arches of the vertebrates, and as a matter of fact it has been 

 suggested that the second pair of arachnoid appendages, so 

 largely developed in the scorpion, do actually represent the 

 mandibular arch, and that the next pair, or perhaps the next 

 two, may represent the hyoid. The vertebrate gills them- 



FIG. 143. Comparison between extinct Crustacean and Vertebrate. 



(A) Gigantostracan, Eurypterus [after NIEDZKOWSKY]. (B) Placoderm, Pterichthys 

 [after NEUMAYER]. 



selves seem, however, more nearly comparable in structure with 

 the gill-plates or gill-books of modern arachnoids, and their 

 more posterior position in those latter is accounted for through 

 a backward migration, since the nerves supplying them are 

 precisely the ones which, for other reasons, have been com- 

 pared with Vagus elements. 



Perhaps one of the strong points in this erratic theory lies 

 in the fact that the vertebrates are here derived, not from a 

 primitive segmented form, like an annelid, in which the somites 



