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 antennae. 

 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 cephalo thoracic segment. 



The abdomen is relatively small and weak and usually 

 remains folded beneath the cephalo thorax. 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 antennae 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, pleurobranchiae 

 only being present. Note the enormously elongated epipodite 



