THE CRAYFISH. 



CAMBARUS SP. 



I. External Anatomy. 



COMPARE with the earthworm with regard to regional 

 differences such as anterior and posterior ends, dorsal and 

 ventral sides, head and tail. Are these regions more or less 

 sharply marked? Is there perfect bilateral symmetry? 



Distinguish an anterior region, the cephalothorax, and a 

 posterior, the abdomen. In what ways are they alike? 

 In what different? Is there a definite head? Note the 

 exoskeleton, is it present everywhere? Where is it thickest 

 and hardest, where thinnest? What explanation can you 

 give for these differences? 



1. Abdomen. Is it segmented? If so, how many seg- 

 ments? The last segment is called the telson. Does it 

 differ from other segments? Is free movement of the seg- 

 ments possible? Is it equal in all directions? How are 

 the segments joined together? Are appendages present? 

 Are they present on all segments ? Are they alike ? 



2. Cephalothorax. Is the form like that of the abdomen? 

 Are segments present? Is there any thing to suggest seg- 

 mentation? The shell-like exoskeleton in this region is 

 called the carapace. Are the edges of the carapace free or 

 fixed? To what extent? Are the appendages of this region 

 similar in form? 



3. Sexes. Distinguish the following characters: 



