xii.] THE FRESH- WATER CRAYFISH. 153 



forming a wide oval plate (scaphognathite) 

 which lies at the anterior end of the gill- 

 chamber (n. 7.). 



p. 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. 



f. 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 eyestalks. Short two-jointed 

 appendages representing only the basipodite. 



22. Now work back over the 20 pairs of appendages 

 and compare each with the third 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 



