86 WILLIAM PATTEN. 



The hind brain of Limulus (fig. 48, H. B.) is composed of 

 five typical thoracic neuromeres, each having the following 

 four pairs of nerves : — (1) The mandibular nerves [md. n., 2 — 6) 

 are purely sensory, and consist of two branches which terminate 

 in the gustatory organs in the mandibles. They arise from the 

 neural surface of the ganglion at the root of the pedal nerve, 

 and usually possess well-defined swellings that contain a few 

 small ganglion cells scattered through a mass of interwoven 

 fibrous substance. Sections through the appendages of half- 

 developed embryos (fig. 73) show two sensory thickenings, 

 one corresponding to the inner mandible {i. md.) and the 

 other to the outer (o. md.). In figs. 71 and 73 are seen 

 ganglion cells separating from the sense organs to form the 

 ganglia of the mandibular nerves in the adult (fig. 3, md. n. g.). 

 The mandibular nerves correspond to the supra-branchial 

 nerves and ganglia of Vertebrates. (2) The pedal nerve is a 

 mixed nerve arising from a large neural ganglion common 

 to it and the coxal nerve ; it supplies the sense organs and 

 muscles of the walking appendages. In Scorpions it is at an 

 early stage probably connected with the lateral segmental 

 sense organs, but the existence of such a connection could not 

 be demonstrated in Limulus, owing probably to the transitory 

 nature and imperfection of the segmental sense organs. This 

 nerve corresponds to the ventral or branchial nerve of Ver- 

 tebrates, with which it agrees in innervating the only im- 

 portant muscles of the head, namely, those of the visceral 

 arches. All the other head muscles of Arachnids and Ver- 

 tebrates, owing to the complete anchylosis of segments, have 

 disappeared. 



The neural and lateral ganglia of Beard probably correspond 

 respectively to the pedal ganglion and the ganglia to the 

 segmental sense organs of Limulus and Scorpions. The 

 anterior and posterior haemal nerves correspond to the pre- and 

 post-trematic of Vertebrates. 



All the nerves of the hind brain of neuromeres of Limulus 

 agree with those of Vertebrates in being separate, while the 

 same nerves in the trunk of both Vertebrates and Arachnids 



