362 APPLIED BIOLOGY 



pairs ? How many ? Each pair is believed to represent a segment, 

 as is evidently the case in the abdomen ; how many segments, then, 

 compose the cephalothorax ? How many in the whole body? 

 Compare: (1) walking legs, and (2) the mouth-parts (appendages 

 used in feeding). 



Study of Living Crayfish. Observe the habits of a living 

 crayfish in an aquarium. In what directions can it walk? What 

 appendages are used in walking ? Are there differences in the move- 

 ments of the appendages ? Startle the animal and observe its move- 

 ments. Can it swim ? In what direction ? What appendages are 

 used in swimming ? Place the animal on its back and observe what 

 appendages are used in turning itself. 



Organs of Sense. Do you find evidence that the crayfish sees ? 

 Examine an eye of a dead crayfish with a hand-lens, and later ex- 

 amine with a microscope thin sections cut with a razor. Small 

 sacs in the basal segments of each antennule have been called "ears," 

 but it has not been proved that they are organs of hearing. It has 

 been shown that these organs enable the animal to keep balanced and 

 in the proper position when moving. In short, they perform the 

 function of the semicircular canals in the human ear, which "make 

 us feel" uncomfortable when in unnatural positions (e.g., head 

 downward). 



With a bristle or straw touch a living crayfish in various places. 

 Where is it most sensitive ? How does it use the antennas ? 



There is some evidence that the crayfish has the senses of taste 

 iid smell, but it is difficult to devise simple experiments to test these. 



II. Internal Organs of Crayfish 



(D or L) Using strong forceps and scissors, break and cut 

 away the dorsal surface of the carapace of a preserved or recently- 

 chloroformed crayfish. 



Circulatory System. Just under the center of the thorax part 

 of the carapace is a cavity (pericardial chamber) in which lies the 

 whitish heart, connected with which are seven arteries whose branches 

 conduct the blood to all parts of the body. The arteries are best 

 seen after injecting some colored fluid into the heart. Blood 

 distributed to all organs by the arteries is collected in irregular 

 spaces in the body, from these it flows into the tubes of the gills, and 

 thence up to the pericardial chamber. Valves in the heart are ar- 

 ranged so that blood from the pericardial chamber can enter the 

 heart, but it cannot go in the reverse direction. 



