PHYLUM ARTHROPOD A 



223 



FIG. 126. Respiratory organs of Astacus fluviatilis. In A the gill-cover is removed 

 and the gills undisturbed; in B the podobranchiae are removed and the outer 

 arthrobranchiae turned down. a lt antennule; a*, antenna; at> lt first; ab z , 

 second abdominal segment; arb. 7-12, inner arthrobranchiae; arb-^. 7 12, outer 

 arthrobranchiae : ep. J, scaphognathite; plb. 11-13, pleurobranchiae; pdb. 7-13, 

 podobranchs; //. /, first pleopod; 6-/J, thoracic appendages. (From Lang's 

 Comparative Anatomy, after Huxley.) 



lei canals in the stem an external, the afferent branchial 

 vein, and an internal, the efferent branchial vein. 



According to their point of origin, the gills (Fig. 126) are 



