II.] THE CRAYFISH AND LOBSTER. 221 



a. The epipodite ; a delicate chitinous lamina, folded 

 upon itself and pleated posteriorly. 



b. The podobranchia ; made up of a series of gili- 

 filaments set upon a central stem. 



i. Examine the same in transverse section. The lamina 

 of the epipodite and the stem of the gill are con- 

 tinuous. 



Cf. the Lobster : the gill is free of the epipodite, 

 and the gill-filaments, which are very numerous, are 

 arranged in close set parallel series. 



3. Examine the remaining 12 branchiae in situ, each con- 

 sists of a central stem giving off a number of delicate 

 filaments. There is nothing answering to the epipodite. 

 They are arranged in two sets 



a. The arthrobranchice ; attached to the inter- articular 

 membranes of the thoracic appendages, with the 

 exception of the first maxillipede and the last am- 

 bulatory leg. There are eleven of them, the odd 

 one being related to the second maxillipede. 



b. The pleurobranchia 



a. Functional ; a solitary gill, attached to the inner 

 wall of the branchial chamber, immediately above 

 the last ambulatory appendage. 



(3. Vestigial ; two to four delicate filaments (best 

 seen after removal of the arthrobranchiae) at- 

 attached anteriorly to and in a line with a. 



In the Lobster the above are all functional. 

 There are, in that animal, 20 gills on each side ; 

 six podobranchiae and four pleurobranchias at- 

 tached as in the Crayfish, but only ten arthro- 

 branchise there being no such gill related to the 

 second maxillipede. 



