ITS NERVOUS SYSTEM AND SENSE ORGANS. 89 



socket on each side in the suture between the clypeus and the 

 epicranium. The sub-oesophageal ganglion gives off branches 

 to the mandibles, maxilla), and labrum. While, therefore, the 

 supra-cesophageal is largely sensory, the sub-oesophageal gang- 

 lion is the masticatory centre. 



The cesophageal ring is double below, being completed by the 

 connectives and the sub-oesophageal ganglion; also by a smaller 

 transverse commissure, which unites the connectives, and applies 

 itself closely to the under-surface of the oesophagus.* 



Two long connectives issue from the top of the sub- 

 oesophageal ganglion, and pass between the tentorium and the 

 submentum on their way to the neck and thorax. The three 

 thoracic ganglia are large (in correspondence with the important 

 appendages of this part of the body) and united by double 

 connectives. The six abdominal ganglia have also double 

 connectives, which are bent in the male, as if to avoid 

 stretching during forcible elongation of the abdomen. The 

 sixth abdominal ganglion is larger than the rest, and is no 

 doubt a complex, representing several coalesced posterior 

 ganglia ; it supplies large branches to the reproductive organs, 

 rectum, and cerci. 



Internal Structure of Ganglia. 



Microscopic examination of the internal structure of the 

 nerve-cord reveals a complex arrangement of cells and fibres. 

 The connectives consist almost entirely of nerve-fibres, which, 

 as in Invertebrates generally, are non-medullated. The ganglia 

 include (1) rounded, often multipolar, nerve-cells ; (2) tortuous 

 and extremely delicate fibres collected into intricate skeins ; 

 (3) commissural fibres, and (4) connectives. The chief fibrous 

 tracts are internal, the cellular masses outside them. A rela- 

 tively thick, and very distinct neurilemma, probably chitinous, 

 encloses the cord. Its cellular matrix, or chitinogenous layer, 



This commissure, which has- been erroneously regarded as characteristic of 

 Crustacea, was found by Lyonnet in the larva of Cossus, by Straus-Diirckheim in 

 Locusta and Buprestis, by Blanchard in Dytiscus and Otiorhynchus, by Leydig in 

 Glomeris and Telephorus, by Dietl in Gryllotalpa, and by Lienard in a large number 

 of other Insects and Myriapods, including Periplaneta. See Lienard, "Const, de 

 1'anneau cesophagien," Bull. Acad. Roy. de Belgique, 2'- Ser., Tom. XLIX., 1880. 



