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 



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

 the gills undisturbed ; in B the podobranchite are removed and the outer arthrobranehiaa 

 turned down, aj, antennule ; «•>, antenna; ab\, first; ab.i, second abdominal segment; 

 arb. 7 — 12, inner arthrobrarichiae ; arb'. 7 — 12, outer arthobranchias ; ep. 5, seaphognathito ; 

 plb. 11 — 13, pleurobranchiffi ; pdb. 7 — 13, podobranchs ; pi. 1, first pleopod ; 6 — 13, 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, podobranchice or foot-gills, springing from the 

 epipodites of the thoracic appendages, from which they are only 



