98 PERIPHERAL NERVES AND GANGLIA. 



territory of so many other nerves of the same nature, in order to innervate a 

 region so far removed from its origin. It is suggestive of the lateral line nerve of 

 vertebrates, but its origin from the tween-brain region is strongly against such 

 an interpretation. It resembles the large nerve in ganoids and teleosts, the 

 ramus lateralis accessorius, which arises well forward in the head and is distributed 

 to the taste buds of the head, back, tail, and fins. 



The character of the sensory terminals to this nerve in Limulus is unknown. 



III. THE NERVES OF THE METENCEPHALON. 



We have already shown that a certain number of abdominal metameres in 

 arthropods move forward and unite with the thorax, and that there is a great 

 reduction in their size and an obliteration of their external boundaries. The 

 appendages and muscles show a similar reduction, but the corresponding nerves, 

 neuromeres and heart segments are but little changed. In fact, the nerves and 

 neuromeres may be relatively more voluminous or extensive than elsewhere. 

 These metameres constitute the vagus zone and their neuromeres the meten- 

 cephalon. Their nerves may be appropriately called vagus nerves, because, as in 

 the vertebrates, they extend backward into regions to which they did not originally 

 belong. 



LIMULUS. 



Neural Nerves. In Limulus, this region contains two metameres, the chi- 

 larial and the opercular. The tergites of these metameres are still visible in the 

 adult, the chilarial tergite forming a narrow band on the posterior margin of the 

 thoracic shield, the opercular tergite, two wing-like segments on the anterior 

 margin of the branchial shield. The hinge joint between the two shields lies 

 between these two metameres. (Figs. 150-155.) The first entapophysis is 

 formed between the chilarial tergites and the true thoracic metameres. (Fig. 193.) 



The chilaria are without question true appendages. Their early develop- 

 ment is like that of the other appendages, and they have separate nerves, muscles, 

 mesoblastic somites, and gill bars. The chilarial and opercular neuromeres 

 have all the typical nerve elements. They resemble the branchial neuromeres 

 more than the thoracic, although in the adult they are intimately fused with the 

 hindbrain and widely separated from the branchial neuromeres. Their nerves 

 pass out of the occipital foramen of the endocranium together with the spinal 

 cord. (Figs. 70-218.) 



The chilarial nerves arise close together from the posterior neural surface 

 of the accessory brain. They pass out of the endocranium just below the roof 

 of the occipital ring, enter the chilaria and supply their muscles, the adjacent 

 skin, and the numerous gustatory spines on their median side. (Fig. 81, n.n 7 .) 



The opercular nerve follows the same course, and on reaching the operculum 



