550 



ZOOLOGY 



SECT. 



the whole organ is plume-like. The filaments are hollow, and 

 communicate with two parallel canals in the stem an external, 

 the afferent branchial vein, and an internal, the efferent branchial 

 vein. The gill is to be considered as an out-pushing of the 

 body-wall, and contains the same layers a thin layer of 

 chitin externally, then a single layer of epithelial cells, and 



FIG. 436. Respiratory organs of Astacus fluviatilis. In A the gill-cover is removed and 

 the gills undisturbed ; in B the podobranchue are removed and the outer arthrobranchi; 

 turned down. ], antennule ; a- 2 , antenna ; a6j, first ; ah.}, second abdominal segment ; 

 arb. 7 12, inner arthrobranchiaj ;" urb'. 7 12, outer arthobraiichue ; ep. ;>, scaphognathite ; 

 plb. 1113, pleurobranchiai ; pdb. 713, podobranchs ; pi. 1, first ploopod ; <jlo, thoracic 

 appendages. (From Lang's Comparative Anatomy, after Huxley.) 



beneath this connective-tissue, hollowed out for the blood 

 channels and containing gland-cells, which will be referred to 

 presently (p. 551). 



According to their point of origin, the gills are divisible into 

 three sets first, 'podobrancliice or foot-gills, springing from the 

 epipodites of the thoracic appendages, from which they are only 



