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MANUAL OF ELEMENTARY ZOOLOGY 



in the branchiostegite is oxygenated through the thin inner 

 wall of that organ. 



The gills receive different names according to their position. Those 

 which are attached to the epipodites of the limbs are known as 

 podobranchice. Others stand upon the membranes which join the 

 limbs to the body, and are known as arthrobranchice, and a few stand 

 upon the inner wall of the gill-chamber (the side wall of the thorax) 

 above the legs, and are known as pleurobranchice. A podobranchia is 

 found on every thoracic limb, except the first pair of maxillipeds, which 

 have no gills, 1 and the last pair of legs, which have only a pleuro- 

 branchia. Two arthrobranchise, an anterior and a posterior, are found 

 upon each limb that has a podobranchia, except the second maxilliped, 

 which has only the anterior arthrobranchia. Well-developed pleuro- 

 branchise are only found above the legs of the last pair, but in the 

 same position above each leg of the three preceding pairs there is a 

 minute process which represents a gill. The following table shows 

 the position of the gills : — 



Ep = epipodite without a gill. 



R= abortive rudiment. 



Each arthrobranchia has a tree-like structure, consisting of a trunk or 

 axis arising from the body by one end, with numerous short branches 

 or filaments. The two pleurobranchise have the same structure. In 

 the podobranchise the axis is fused to the epipodite along the greater 

 part of its length, so that the filaments appear to arise from the 

 epipodite. The tip of the gill, however, stands free. The epipodite 

 itself is a long plate with a wide base, a narrower stem, and at 

 the end a second expansion, the lamina. The stem and lamina are 

 folded along the length of the epipodite, so that a groove is formed, 

 into which fits the gill of the limb next behind. 



1 In A.fiuviatilis they have each a vestigial arthrobranchia. 



