LESSONS IN ZOOLOGY. 269 



bending it backward and forward several times, notice bow tbe 

 segments are jointed together. With a pair of scissors or a 

 knife separate one of the segments of the abdomen from the 

 others, and notice the upper part or tergite, the lower part 

 or sternite, the side piece or pleurite, and the two appendages 

 or swimmerets. Compare the segment studied with other seg- 

 ments. 



Observe the tail fin; the middle portion is the telson, the side 

 lobes belong to the sixth joint of the abdomen. The abdomen 

 of the female is relatively wider than that of the male, and the 

 genital openings of the male are on the first joints of the last pair 

 of thoracic appendages, while in the female they are on the first 

 joints of the last thoracic appendage but two. The eggs are 

 usually carried on the under side of the abdomen, glued to the 

 swimmerets. 



Placing the crayfish on its back, count the legs and other 

 appendages of the cephalothorax and note if any of them appear 

 to arise from segments. How many joints in each leg, how do 

 the legs differ from each other, and how do the claws differ from 

 the legs. Notice also the three pairs of peculiar appendages an- 

 terior to the claws or chela? ; they are called foot jaws ; test them 

 carefully so as to learn how they work. Compare the foot jaws 

 with each other and with the legs and claws. 



Now remove the legs, claws, and foot jaws from one side, using 

 care not to tear other parts. Then turn back or break away the 

 free edge of the carapace on that side, and the white feathery 

 organs brought to view are the gills, of which there are eighteen 

 pairs; six are attached to the second and third foot jaws, the 

 chela?, and the first, second, and third pairs of legs, the remain- 

 ing are fixed to the sides of the body. Each consists of a central 

 stem bearing a number of delicate filaments. Separate each 

 pair of gills and find out to what it is attached and how it is at- 

 tached. Study carefully the chamber containing the gills and 

 note the parts which compose it. With a probe explore for open- 

 ings from the branchial chamber. Projecting forward there is a 

 well-marked canal ; in this there is a flat oval plate, attached to 



