XII.] THE FUESH-WATER CEAYFISH. 147 



@. Anterior maxilla: epipodite and exopodite 

 undeveloped : the endopodite foliaceous. 



e. The mandible. Its strong toothed basal joint 

 (protopodite) bearing a small appendage (the palp) 

 which represents the endopodite ; the epipodite 

 and exopodite unrepresented. 



/. The antenna. Its two-jointed basal portion (proto- 

 podite) bearing a flattened plate (the rudimentary 

 exopodite} and a long multiarticulate filament (the 

 endopodite) : the opening of the green gland (17) 

 on the oral side of the basal joint of the proto- 

 podite. 



g. The antennula. Its large trigonal basal joint 

 (protopodite), bearing a pair of jointed filaments 

 (endopodite and exopodite} : the opening of the 

 auditory organ (24) in the midst of a minute 

 hairy tuft on the basal joint. 



h. The ophthalmites or eye-stalks. Short two-jointed 

 appendages representing only the basipodite. 



22. Now work back over the 20 pairs of appendages 

 and compare each with the second maxillipede: all 

 may be supposed to be derived from it by suppression, 

 coalescence or special change of form ; it is what is 

 called a typical appendage. 



23. Structure of the Eye. 



a. Take the eye of a lobster which has lain four or 

 five days in 0'5 per cent, solution of chromic acid 

 and then twenty-four hours or more in alcohol. Ex- 

 amine its surface with one inch obj. with reflected 

 light. It will be seen to be marked out into a 



102 



