CRAYFISH 103 



much the same. The maxillipeds are more or less covered with hairs, 



some of which are believed to serve as chemical sense organs and 



others as tactile sense organs. The maxillipeds aid in holding the 



food. (W. f. 64.) 



3. Head Appendages 



Segments v and vi each bears a pair of appendages, known as the 

 first and second maxillae. The second maxilla, on segment vi, 

 is one of the most highly modified appendages of the entire body. 

 The protopodite is quite unusual and consists of thin, pliable 

 plates. The exopodite has become greatly modified and, together 

 with the epipodite, forms a broad, flat structure, the scaphogna- 

 thite, which, by its movements in the gill chamber, keeps a cur- 

 rent of water passing over the gills. The endopodite is a small 

 filament consisting of only one segment. The first maxilla, on 

 segment v, is one of the smallest appendages and consists of the 

 protopodite and the endopodite which is a small, atypical struc- 

 ture. The exopodite is entirely lacking. Segment iv bears a pair 

 of jaws, or mandibles. Each mandible consists chiefly of a proto- 

 podite which is made up of two segments, and to which very heavy 

 muscles are attached. The exopodite is lacking and the endo- 

 podite is greatly reduced. The mandibles are primarily used for 

 crushing the food. (W. f. 64.) 



Segments ii and iii bear a pair of antenntjles and antennae 

 respectively. The antennae are considerably larger than the 

 antennules. The protopodite of each antenna is made up of two 

 segments. The proximal segments bear an opening of an excretory 

 organ known as the green gland. The exopodite of the antennae 

 is a short, thin, blade-like structure, which projects laterally, while 

 the endopodite is a long, many-jointed, whip-like structure. The 

 antennules have a protopodite which is made up of three segments, 

 the proximal one of which contains a sensory organ, known as the 

 statocyst. The exopodite and the endopodite are practically of 

 the same length and, although somewhat smaller, have the same 

 structure as the endopodites of the antennae. The antennules 

 and antennae are primarily sense organs. 



AUTOTOMY AND REGENERATION OF THE APPENDAGES. Many of 



the Crustacea, including the Crayfish, possess the power of casting 

 off an injured appendage. This is known as autotomy. There 

 is a definite breaking joint between the second and third segments 

 of the walking legs, and when these appendages are injured, the 



