DISSECTION OF CRAYFISH. 159 



either side of which is a strong jaw (mandible). How do 

 these jaws move in comparison with those of man ? Take 

 one out, and see of how many joints it is composed. 



The cephalothorax is covered above by a large continu- 

 ous plate, the carapax.* Does this show signs of seg- 

 ments. With the forceps lift the hinder corner of the 

 carapax on the side where the mouth-parts still remain, 

 and see where it joins the body. Then with the scissors 

 cut away the free portion, thus laying open the gill- 

 chamber, and exposing the body- wall and the numerous 

 gills or branchiae. Are any of these attached to the legs or 

 to the body-wall ? Move the maxillae, and see the opera- 

 tion of the gill-bailer. Can water obtain free access to the 

 gills ? 



In front of the mouth occur the "feelers" or antennae. 

 Could these be compared to the legs ? Can you find exop- 

 odite or endopodite in them ? Examine the basal joint 

 of the larger or posterior one (the antenna proper), and 

 find an opening, the outlet of the green gland (see below). 

 Is it in any way comparable in position to the reproductive 

 opening ? In the smaller feelers (antennulae) look for the 

 ear-sac on the upper surface of the basal joint. (It is 

 covered with a thin membrane, around which hairs are 

 arranged.) Above the antennulae are the eyes. Are they 

 movable ? Examine the black portion (cornea), and see 

 the small portions (facets) of which it is composed. 



Make a tabular arrangement of the appendages of the 

 body,f and ascertain by Savigny's law (p. 158) how many 

 segments there are in the body of the crayfish. Compare 

 the segments, and see how their diversity is brought about 



* Often written carapace. The spelling here adopted is preferable. 

 f For reasons which cannot be discussed here, the eyes are not re- 

 garded as appendages comparable to the others. 



