550 



ZOOLOGY 



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

 communicate with two parallel canals in the stem — an external, 

 the ajfcrent branchial vein, and an internal, the cff'crvnt branchial 

 rein. 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. 4'M}. — Respiratory organs of Astacus lluviatilis. In A the gill-covor is removed and 

 the gills undisturbed ; in B the podobranchi;^ are removed and the outer arthrobranchite 

 turned down, aj, antennule ; ao, antenna; abi, first; a/>o, second abdominal segment; 

 arb. 7 — IS, inner arthrobranchise ; arb\ 7 — IS, outer arthobranohi^ ; ep, 5, scaphogiiathite ; 

 jilh. 11 — t.j, pleuvobranchipe ; juib. 7 — IS, podobranehs ; -pi. 1, first pleopod ; 6' — 13, thoracic 

 appendages. (From Lang's Comparative Anatoriyi, 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, podobranchiee or foot-gills, springing from the 

 epipodites of the thoracic appendages, from which they are only 



