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. 
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 1. 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. They are not prominent in the crayfish. 
