A CRAB 43 
gill-chamber along this edge as it does in the crayfish and the 
lobster. An opening is present, however, at the base of the 
cheliped through which the water enters. Pass a probe into 
the branchial chamber through this opening. Notice the 
prominent stalked eyes; also the two pairs of delicate antenne. 
Examine and identify the mouth-parts and the thoracic legs; 
they will be found to correspond to those of the crayfish or 
the lobster. Find the openings of the genital organs; in the 
male on the ventral surface of the last and in the female of the 
antepenultimate cephalothoracic segment. 
The abdomen is relatively small and weak and usually 
remains folded beneath the cephalothorax. It lacks the swim- 
ming fin; most crabs cannot swim. The common blue crab, 
however, swims very well by means of the fifth pair of perio- 
pods. The number of abdominal segments is variable, fusion 
having taken place between certain of the somites. This num- 
ber is also not the same in the male and the female of the same 
species. Raise the abdomen from the cephalothorax and observe 
the swimmerets on its ventral surface. In the female note the 
long chitinous hairs which fringe the swimmerets. It is to 
them that the eggs and newly born young are attached. The 
only swimmerets present in the male are the first two pairs, 
which are functional in pairing. 
Exercise 1. Draw a dorsal view of the animal with the abdomen 
extended, being careful not to omit the antenne and the 
eyes, and label all the parts observed. 
Exercise 2. Construct in your notebook a table showing the 
relation of the appendages and somites similar to that made 
use of with the lobster or the crayfish. (See page 35.) 
The gills. With stout scissors cut off the right branchios- 
tegite and expose the gills. These will be found to be quite 
different from those in the lobster or the crayfish, pleurobranchie 
only being present. Note the enormously elongated epipodite 
