68 LABORATORY WORK. 



Compare the somites behind the third with that one. 

 Do all have the two-branched appendages? How are the 

 swimmerets of the sixth somite modified? How does the 

 last somite (tdsori) differ from the others? Where is the 

 vent? Compare the appendages of the first and second 

 abdominal somites with those of the third. In the male 

 they are peculiarly modified. What numerical relations 

 do you find between somites and appendages in the abdo- 

 men? (Savigny's law). 



Examine the lower surface of the cephalothorax and 

 see if you can find traces of somites, especially in the 

 region near the abdomen. How many appendages on 

 one of these somites? How many pairs of large legs, 

 including the 'pincers' (chelae), do you find? In the hinder 

 pair of legs how many joints do you find? Can you 

 distinguish exopod and endopod? Compare this leg, 

 joint by joint, with the big claw. What change would 

 make it into a chela? How many of these legs are chelate? 

 Look on the inside of the basal joints of the legs for open- 

 ings (outlets of the reproductive organs). If they occur 

 on the middle pair the specimen is a female; if on the last 

 pair it is a male. What is the sex of your specimen? 



Study the appendages (mouth-parts) in front of the big 

 claws. In order to do this properly it will be necessary 

 to remove those of one side one by one, by grasping the 

 base of the appendage with the forceps and pulling it out. 

 Be careful to get all of each appendage, and nothing else. 

 The three hindermost (or outer) mouth-parts are the jaw- 

 feet (maxillipeds) . Compare the hinder pair with the 

 third swimmeret. Do you find basiopod, exopod, and en- 

 dopod? Compare it with one of the walking-legs. Which 

 part, exopod or endopod, is lacking in the latter? Draw 

 each of the maxillipeds. 



