A CRAYFISH OR A LOBSTER 31 



in the largest lobsters are powerful enough to crush a man's 

 arm. Note the difference between the right and the left claw, 

 If any. Back of the chelipeds are four pairs of walking legs, 

 [n the male animal the paired external openings of the genital 

 Drgans are at the base of the last pair of walking legs, in the 

 female at the base of the antepenultimate pair. Find them. 



The abdomen. The seven somites forming this body-division 

 ire all free and jointed with one another. Note the difference 

 in the thickness of the cuticula on the dorsal and the ventral 

 surfaces, also its thinness at the joints. The appendages on 

 ;he abdomen have various uses. They probably have a general 

 respiratory function. In the male the first two pairs are 

 functional in pairing, in the female the first five pairs hold 

 ;he eggs from the time they are laid until the young are 

 latched. The last pair in both sexes is large and broad 

 md with the end-segment forms the swimming fin. The end- 

 segment is called the telson ; it bears no appendages ; the anal 

 jpening is in its ventral side. 



The natural color of the animal is usually a greenish black, 

 3ut hot water or alcohol turns it red. 



Exercise l. Draw an outline of the dorsal side of the animal 

 and label all the parts. 



Cut off the right branchiostegite with the scissors, taking 

 jare not to injure the gills beneath. Push aside the gills and 

 lotice the thin integument which forms the lateral wall of the 

 jephalothorax. Observe the method of attachment of the gills, 

 rhey are feathery, thin-walled expansions of the body-wall and 

 ire attached either to it or to the basal portions of the legs, 

 rhey present a very large surface to the surrounding water, and 

 ;he blood circulating through them is thus oxygenated. Notice 

 ;he epipodites, the skinny flaps which project from the basal 

 oints of many of the legs and separate the gills of a segment 

 :rom those of the next. They are not prominent in the crayfish. 



