140 



THE ORIGIN OF VERTEBRATES 



the entapophysial ligament, on the contrary, just like muco-cartilage, 

 takes on an intense purple colour when stained with thionin. It 

 possesses a mucoid substratum, just as does muco-cartilage, and in 

 both cases a perfectly similar soft cartilage is born from it. 



One difference, however, exists between the branchial cartilages of 

 these two animals ; the innermost axial layer of the branchial bar of 



Fig. 59. — Diagram of Limulus, to show the Nerves to the Appendages (1-13) 



and the Branchial Cartilages. 



The branchial cartilages and the entapophysial ligaments are coloured blue, the 

 branchise red. gl., generative and hepatic glands surrounding the central nervous 

 system and passing into the base of the flabellum (fl.). 



Limulus is very apt to contain a specially hard substance, apparently 

 chalky in nature, so that it breaks up in sections, and gives the 

 appearance of a broken-down spongy mass ; if, however, the tissue is 

 first placed in a solution of hydrochloric acid, it then cuts easily, and 

 the whole tissue is seen to be of the same structure throughout, the 

 main difference being that the capsular spaces in the axial region 

 are much larger and much*more free from cell-protoplasm than are 

 those of the smaller younger cells near the periphery. 



