THE CRAYFISH 111 



(a) Male. The first abdominal appendages are modified 

 into tube-like or spine-like organs; the abdomen is narrower 

 than the thorax; the genital openings are on the bases of 

 the hindmost legs. 



(6) Female. The abdomen is slightly broader than the 

 cephalothorax; the genital ducts open on the bases of the 

 third pair of legs. What is the character of the first abdom- 

 inal appendages? 



4. External Openings. (a) auditory, on the dorsal side 

 of the basal joint of the antennules; ('&) excretory, at the 

 base of the antennae on the ventral side; (c) mouth, on the 

 ventral side between the jaws; (d) genital ducts already 

 noted; (e) anus, on the ventral side of the last segment of 

 the body. 



Draw the entire animal from the side, X 2. 



II. Appendages. 



1. Abdominal. Carefully dissect the appendages from 

 one side of the abdomen and arrange them in order on a 

 piece of paper. Using the third appendage as a type notice 

 its biramous character ; there is a basal portion, the pro- 

 topodite, and two distal portions, the exopodite and the 

 endopodite. Compare the other appendages with this one 

 and determine whether they have the same structure. Are 

 there any variations from this structure? What are they? 

 Which appendages show the greatest modification? In 

 what way? Can you explain it? Draw the first, the third 

 and the last appendage. 



2. Cephalothoracic. In a similar manner dissect off the 

 walking appendages from the same side of the animal and 

 arrange them in order as before. Explain such differences 

 in structure as appear. Do these appendages bear any 



