THE CRAWFISH. 113 



see how many feet it walks with, and in what order they 

 are moved. 



Startle it by thrusting a stick toward its eye, and see 

 its rapid locomotion backward. How is this effected ? 



Observe its feeding habits. 



Place a live crawfish in a small dish, and barely cover 

 it with water. Take hold of its body with forceps, and 

 observe its way of defending itself. 



Observe that its eyes are stalked and capable of pro- 

 trusion outward. 



Gill Currents. Observe that on each side of the body 

 that portion of the hard crust beneath which the legs are 

 attached is marked off from the rest by a shallow curved 

 groove. Within the portion so marked off at the base of 

 the legs is the gill chamber, containing true gills. This 

 chamber is open at both ends for the passage of currents 

 of water. Determine the direction of the currents by 

 placing a drop of ink or other colored fluid in the water 

 near each end of the chamber, and watching it move. 



Study the structure of a crawfish which has been a 

 short time in alcohol. 



Plan of Structure. The body is divided into two 

 regions. The anterior one is the cepTialothorax ; the 

 posterior one, the abdomen. 



Observe that the body is made up of a series of seg- 

 ments. These segments are very evident in the abdo- 

 men, but not so plain in the head and thorax, where they 

 will have to be looked for on the ventral surface. It will 

 be seen that each pair of legs, at least, is attached to a 

 single segment, though no traces of segments may be dis- 

 coverable on the dorsal surface. 



Observe that every distinguishable segment, except the 

 hindmost, bears on its lower surface a pair of appendages, 



NEED. ZOOL. 8 



