276 AN INTRODUCTION TO THE STUDY OF FOSSILS 



bases toothed for passing the food obtained by them forward 

 to the mouth. The abdomen consists of seven segments, each 

 of which except the last, or telson, bears a pair of appendages. 



In the female the five 

 anterior pairs are small 

 swimming feet or pleo- 

 pods, w^hile the sixth pair 

 is much enlarged, each 

 consisting of two flat- 

 tened parts which with 

 the telson compose the 

 live-parted tail-fin, the 

 principal organ of loco- 

 motion. The male dif- 

 fers only in having the 

 anterior two pairs of 

 pleopods converted into 

 incomplete tubes for 

 transferring the sperma- 

 tozoa to the body of the 

 female. 



The typical appendage 

 consists of one or more 

 basal joints, the proto- 

 podite, which divides 

 into two branches : an 

 inner and ventral, the 

 endopodite, and the outer 

 and dorsal, theexopodite ; 

 this latter division is 

 largely respiratory in 

 function. 



Fig. 123. — The crayfish, Cambariis hartoni 

 Fabr. ( X I), from Colchester, Vermont. Dor- 

 sal view; abd., abdomen; ant. i, anterior an- 

 tennae ; ant. 2, posterior antennae ;ceph., cephal- 

 othorax; e., eye; J.j., foot jaws; g.c, gill- 

 cover; pin., movable arm of pincer; tel., 

 telson ; w.l., walking legs (five pairs) ; XIV, 

 fourteenth body segment or first abdominal 

 segment ; XIX, nineteenth body segment. 



The skeleton is exter- 

 nal and is both protec- 

 tive and supporting to 

 the muscles and other 

 soft parts w^ithin. It is 

 composed of chitin se- 

 creted by the epidermis, 

 and hardened with more 



