i 4 NERVOUS SYSTEM AND SENSE-ORGANS 



ganglia are quite distinct, and smaller than those of the thoracic 

 region, in correspondence with the relatively small size of their 

 segments. But the last two pairs have united into a somewhat 

 larger nerve-mass, which supplies the last two segments of the 

 body, that include the large tail-fin. It has been shown by ex- 

 periment that the brain of the Crayfish is the dominating centre 

 of the nervous system, while the remaining nerve -masses are 

 centres of reflex action for the segments which they supply. 



Turning now to the Crab, in which the head and thorax are 

 relatively short and broad, and the abdomen insignificant, the 

 brain is comparable to that of a Crayfish, but 

 all the ganglia of the short ventral cord have 

 fused together into a single mass, placed near 

 the under side of the thorax, and perforated by 

 the artery which runs down from the heart (fig. 

 1016). 



In all the three Crustaceans described there 

 is a visceral nervous system, the roots of which 

 are indicated in the figures. 



Nervous Systems of Air-breathing Arthro- 

 pods (Tracheata). Comparison of Peripatus, 

 Myriapods, Arachnids, and Insects will show 



end of the nerve-ring while filSt jjjg g ame ]j nes afe flowed aS itt OtlS- 



the large ventral ganglion is 



seen behind, with numerous taceans. In the less-specialized forms, where 



radiating nerves. . 



the body is elongated and there has been but 

 little fusion between segments, the nervous system is very like 

 that of an Annelid. But in cases where the body is compara- 

 tively short and much fusion has taken place, the nervous system 

 is concentrated to a corresponding degree. 



We have already had occasion to see that Peripatus is the 

 most primitive of all living air-breathing Arthropods, and has 

 the closest affinity to Annelids. This view is fully borne out by 

 examination of the nervous system. The upper side of the nerve- 

 ring is swollen into a relatively large brain, and the two halves 

 of the ventral cord are widely separate, though united by numerous 

 transverse bands of nerve-fibres. The outer part of each cord, 

 through its entire extent, contains numerous nerve-cells, and these 

 are not aggregated into well-marked ganglia (fig. 1017). 



Myriapods, such as ordinary Centipedes and Millipedes, are 

 rather more specialized than Peripatus, and possess a well-marked 



