THE CRAYFISH 



247 



is passed through a similar opening upon the last leg. Of 

 the abdominal limbs the first and second pairs are best 

 studied after the third, fourth, and fifth. The latter are 

 alike and consist each of a short coxopodite, a long basi- 

 podite, and an endopodite and exopodite each composed of 

 a number of imperfectly separated joints, of which the first 

 is longer than the rest. The endopodite is rather longer 

 than the exopodite, and both bear numerous, plumed 

 bristles. The second abdominal limb of the female is like 

 those behind it. In the male the first joint of the endopo- 

 dite is much elongated and bears at the end on the outside 

 a thin plate rolled into a scroll. The first abdominal limb 

 has no exopodite in either sex. In the female it is minute. 

 In the male the basipodite and endopodite are fused, 

 flattened, and rolled scrollwise into a tube for conveying 

 sperm to the female. The limbs of the last {sixth) 

 abdominal pair have short, undivided protopodites, and 

 very broad endopodites and exopodites, the former of one, 

 the latter of two joints. They are directed backwards, and 

 form with the telson the tail fan used in swimming. 



Head 



Thorax 



Table of the Segments of the Crayfish 



1. Preantennal limbless segment 



2. Antennules . . . 1. 

 J- aensory 



Abdomen 



3. Antennae (11) 



4. Mandibles . 



5. Maxillules . 



6. Maxillae (11) 



7. Maxillipeds (1) . 



8. „ (11) • 



9. „ (in) . 



10. Chelipeds . 



11. Walking legs (1) . 



12. „ „ (11) ? 

 13- .. .. ( ra > 



U4- » » ( IV ) i 



"15. Abdominal limbs (1) 

 16. „ „ (II) 



17- » .. (i«) 



18. „ ,, (iv) 



19- .. „ ( y ) 



20. „ „ (vi) 



Telson 

 9 Female opening. 



limbs. 



Jaws. 



•Legs. 



VPaddles. 



Uniramous limbs. 



)' 



[•Tail fan. 

 ij Male opening. 



