PHYLUM ARTHROPODA 



227 



stantly moving in one direction, viz., 

 from the heart by the arteries to the 

 various organs of the body, where it 

 receives carbonic acid and other waste 

 matters ; thence by sinuses into the 

 great sternal sinus ; from the sternal 

 sinus by afferent branchial veins to the 

 gills, where it exchanges carbonic acid 

 for oxygen ; from the gills by efferent 

 branchial veins to the branchiocardiac 

 veins, thence into the pericardial sinus, 

 and so to the heart once more. 



The nervous system (Fig. 129) con- 

 sists of a brain (g) and a ventral nerve- 

 cord, united by cesophageal connectives 

 (sc). The ventral cord is double, but the 

 right and left halves have undergone 

 partial fusion, so that the ganglia, and 

 in the abdomen the connectives also, 

 appear single instead of double. The 

 ventral cord contains twelve of these 

 ganglia, the first js infra-cesophageal, 

 being larger than the others and formed 

 by the union of the ganglia belonging 

 to the last three cephalic and first three 

 thoracic segments. All the remaining 

 segments have their own ganglia, with 

 the exception of the telson, which is sup- 

 plied from the ganglion of the preced- 

 ing segment. There is a visceral system 

 of nerves (s) supplying the stomach, 

 originating in part from the brain and in 

 .part from the cesophageal connectives. 



FIG. 129. Nervous system 

 of Astacus fluviatilis. 

 bg, sub-oesophageal gang- 

 lion; cs, commissural 

 ganglion; g, brain; s, 

 visceral nerve; sc, oe- 

 sophageal connective; y, 

 post-oesophageal commis- 

 sure; IV-V1II, thoracic 

 ganglia; /-6, abdominal 

 ganglia. (From Lang's 

 Comparative Anatomy, 

 after Vogt and Yung. ) 



