SOME REACTIONS AMONG ANIMALS 165 



side of the mouth are called the mandibles. See any good figure or chart 

 if no mounted mouth parts are available. 



4. In what directions must the jaws move in biting food ? 



5. For what are the large claws fitted ? 



e. Breathing 



Method. Place crayfish in a tray with about one inch of water in it, 

 and drop with a pipette a little carmine mixture or red ink between the 

 bases of the walking legs. Or hold the specimen ventral side up, and do 

 likewise, and replace in tray. 



Observations. 1. Tell just what was done in performing the experi- 

 ment on respiration. 



2. Did you observe any currents of colored water ? If so, where ? 

 How many ? 



3. Did the currents flow steadily or in jets, if seen ? 



4. Now note the location of the gills in a specimen with part of the shell 

 removed, prepared by the instructor. 



Conclusions. 1. What seems to be the cause of the colored currents of 

 water ? Note. There are small paddle-like structures called gill bailers, 

 that drive the water ahead. 



2. Tell any reasons why you think the streams did not come out of the 

 mouth. 



3. What do you conclude is the course of the water that circulates 

 through the gills ? Where does it enter ? Where leave ? 



Drawing. Make a sketch showing the directions of the currents 

 of water as they enter and leave the cavity containing the gills (gill 

 chamber). 



f. A Morphological Study (For Advanced Students) 

 1. GENERAL CHARACTERS OF THE SHELL 



Observations. 1. What is the general shape of the body of a cray- 

 fish ? 



2. Note that the body is divided into two distinct parts or regions. 

 The urisegmented anterior region is called the cephalothorax, while the 

 segmented posterior region is called the abdomen. 



3. Is the cephalothorax all in one place? The transverse groove 

 separating the head proper from the thorax is called the cervical groove. 



4. How many segments do you find in the abdomen ? 



5. Does the shell cover the entire body ? In answering this question 

 note carefully the joints. The shell is called the exoskeleton. 



