34 PHYLUM ARTHROPODA 



the lobster. Find the openings of the genital organs ; in the male 

 on the ventral surface of the last cephalothoracic segment and in 

 the female in the third segment from the last. 



The abdomen is relatively small and weak and usually remains 

 folded beneath the cephalothorax. It lacks the swimming fin ; 

 crabs of most species cannot swim. The common blue crab, how- 

 ever, swims very well by means of the fifth pair of periopods. 

 The number of abdominal segments is variable, fusion having 

 taken place between certain of the somites. This number 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 newborn 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 26.) 



The Gills. With stout scissors cut off the right branchiostegite 

 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 of the 

 first maxilliped which extends across the gills to the hinder part 

 of the branchial chamber. 



Exercise 3. Construct a table showing the relation of the gills to the 

 somites similar to that made use of in the dissection of the lobster 

 or the crayfish. (See page 27.) 



Exercise 4. Draw a diagrammatic cross section representing an out- 

 fine of the body wall in the region of the walking legs ; show in this 

 the relation which branchiostegites, legs, and gills bear to the body. 



Internal Anatomy. With strong scissors and forceps remove 

 the shell from the entire dorsal surface of the body, taking care 



