VAGAL AND HYPO-BRANCHIAL NERVES. 



107 



displacement and condensation that we have just described in the haemal ends of 

 the hypo-branchial muscles. 



This condition is a very ancient one, for these very muscles, in this position, 

 no doubt cause that folding of the thorax onto the abdomen which is so common 

 in trilobites. I have seen the same thing in Bunodes, very much to my astonish- 

 ment. For sections of specimens that appeared to be headless, showed that the 

 cephalo-thorax was present, but doubled over so as to lie with its neural surface 

 flat against the neural surface of the branchial region. 



The general trend of the branchio-thoracic nerves no doubt has been deter- 

 mined by these morphological changes in the muscles; but the union of these 

 several nerves into a common trunk is to be regarded as an expression of the 

 tendency to gain simplicity by the merging of several separate agents, performing 

 the same function, into a single one. 



V. Relation of the Vagal and Hypo-branchial Nerves in Arachnids to 



THOSE IN Vertebrates. 



The entire system of nerves belonging to the vagal and branchial regions in 

 the arachnids, represents the initial stages in the evolution of the vagal and bran- 

 chial complex in vertebrates. We can already distinguish in the arachnids the 

 beginning of that remarkable segregation of 

 similar components into compound nerves, 

 that in the vertebrates has given rise to the 

 branchial, hypoglossal, cardiac, visceral, and 

 lateral line nerves; and the beginning of that 

 readjustment in the position of the terminals 

 that has given to each set of components their 

 characteristic distribution and direction of 

 growth. (Figs. 57, 58.) 



The branchio-thoracic muscles and nerves 

 of Limulus are clearly comparable with the 

 hypoglossal nerves and muscles of vertebrates. 

 In both vertebrates and arachnids, the nerves 

 arise: a. from a large number of post-vagal 

 neuromeres (five branchial and one oper- 

 cular); b. they are either haemal nerves (ven- 

 tral roots) or branches of haemal nerves ; c. they 

 are united to form a compound longitudinal 

 trunk, terminating in an extensive plexus; d. the distal ends of both muscles and 

 nerves migrate a long distance forward onto the anterior haemal surface of the 

 head, thus causing the nerves to follow their characteristic U-shaped course, and 

 disguising the original relation between the metameric arrangement of organs on 

 the neural and haemal surfaces; e. the distribution of the neural nerves (branchial 

 arch nerves) is not affected by these changes. 



Fig. 81. — Muscles and nerves of the chi- 

 laria of Limulus, from anterior side. The ap- 

 pendages are revolved outward about 45°. 

 b.cJ, Capsuliginous bar, or branchial cartilage 

 of the chilaria. 



Muscles: 70-^, Plastro-coxal; 7 / and g, 

 tergo-coxal; i.m., inter-tergal; up..m.7, veno- 

 pericardiac. 



