A CRAYFISH OR A LOBSTER 25 



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. 

 In the male animal the paired external openings of the genital 

 organs 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 

 are 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 

 the 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 

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

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

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

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

 opening is in its ventral side. 



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

 but 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 

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

 notice the thin integument which forms the lateral wall of the 

 cephalothorax. Observe the method of attachment of the gills. 

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

 are attached either to it or to the basal portions of the legs. 

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

 the blood circulating through them is thus oxygenated. Notice 

 the epipodites, the skinny flaps which project from the basal 

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

 from those of the next. 



