xxxii Journal of Comparative Nei'rology. 



(vsophagcal lobes, and an infra-oesophageal commissure, which traverses 

 the pharyngeal collar. From the antennarj lobes a pair of nerves pass 

 to the second pair of antenn;v; from the (Esophageal lobes nerves pass to 

 the root of the first unpaired visceral ganglion (g. stomato-gastrique) 

 and to the labium. Among insects the third somite and its ganglion 

 sufter great reduction. The somite does not bear appendages, and the 

 antennar\- nerves, with their lobes, have disappeared. However, the 

 representative of the infra-oesophageal commissure remains. It is united 

 to the brain, and gives off a nerve to each root of the first visceral gan- 

 glion and to the labium. From the first visceral ganglion arises an 

 important nerve, which during its course sufi'ers several I'cal ganglionic 

 swellings. In the Crustacea this is called the visceral nerve, while in 

 the insects it is called the recurrent. In the iiiyriofods the tritocerebron 

 consists of the same elements as the corresponding portion of the insect 

 brain. In the ChiJognathes and Scntigera the commissure traverses the 

 pharyngeal collar, while in the ot'rer Chilofodes it is situated in the 

 brain itself. In most families the ganglia consist eitlier of two small 

 bodies situated one at each extremity of the transverse commissin-e 

 {^iilides, Gloiiieridcs) or of a ganglionic mass situated upon the ventral 

 surface of the commissure [Sciitigcres. Lif/iobiidcs). Between these 

 two masses, and isolated from the remainder of the brain, there is a 

 median longitudinal nervous mass, from which arises the visceral nerve. 

 This suppression of the first visceral ganglion and the substitution of a 

 portion of the brain for it is a noteworthy fact. If we admit that the 

 myriopoda are inferior to the Crustacea, then this fact warrants the sup- 

 position that an isolated visceral ganglion is a secondary feature, and 

 that originally the visceral nerve had its origin in the brain itself. A 

 study of the visceral nerve strengthens this belief. Usually the visceral 

 nerve consists of fine fibres, resembling those of the neuroglia, at whose 

 origin several ganglionic cells are located. In the Scitfig-era, however, 

 this nerve is composed of the neuroglia itself, and contains several nerve 

 cells. It is no longer a nerve, but an elongated ganglion, the homo- 

 logue of the visceral nervous system of the Crustacea and the insects. 



4. AracJinida. — Among the Araclniida the brain consists of three 

 ganglia, corresponding to three somites; but only two of these are pre- 

 oral the third (the mandibular ganglion) is post-oral. The two pre-oral 

 ganglia and their concomitant somites correspond to the first and third 

 ganglia and concomitant somites of the insecta, myriopoda, and Crus- 

 tacea. No trace of a homologue of what constitutes the deutocerebron 

 of these latter arthropods is found in the Araclniida. This implies the 

 absence of the second cephalic somite, the somite which bears the first 

 pair of antennae of the Crustacea^ and the antenna* of the Insecta and 

 Myriopoda. 



The optic ganglion is divided into three parts; the optic niduli, the 

 posterior stratified body, and the cerebral niduli. In the Araiiridtr the 

 structure of the cerebral nidulus is c|uite simple, but in the Scor/'io/zidt/- 



