316 



ZOOLOGY. 



are enclosed in a sort of gill-chamber by the branchiostegite. 

 During life the scaphognathite is continually working 

 backwards and forwards in a rapid and vigorous manner, 

 and so producing a current of water forwards through the 

 gill-chamber. Thus fresh oxygen is continually being 

 brought in solution, and the carbon dioxide carried away. 



21. Renal Excretion Green Glands. The renal-ex- 

 cretory organs occupy a position not seen in any of our 

 other types, viz. in front of the mouth. There is a single 



pair, each consisting 

 of a gland, green in 

 colour (when fresh), 

 from which a thin- 

 walled, distensible, 

 bladder - like tube 

 leads to the external 

 opening on the 

 "papilla of the green 

 gland " on the basal 

 joint of the antenna 

 (fig. 158). 





Fig. 158.- GREEN GLAND AND NEIGHBOURING 

 PARTS OF ASTACUS. 



The protopodite of the antennule should have three 

 'oints, not four. 



22. The Nervous 

 System (fig. 159) re- 

 sembles that of the 

 earthworm more 

 nearly than any 



other we have seen. It consists of a pair of cerebral 

 or supra-oesophageal ganglia in an antero-dorsal position, 

 a metameric series of ganglia along the ventral side of 

 the thorax and abdomen, the first of which is united to 

 the cerebral ganglia by a pair of circumoesophageal 

 commissures like those of Lumbricus. But the ventral 

 part is not a continuous nerve-cord as in Lumbricus 

 it is a chain of separate ganglia, joined by longitudinal 

 commissures. It shows (as does that of Lumbricus in 

 its development) a distinctly paired structure, as though 

 two lateral series of ganglia had fused in the middle 

 line. This is most clearly seen in the hind part of the 



