THE CRAYFISH 99 



brane surrounded by bristles called setae. This is tHe en- 

 trance to the ear sac, the supposed organ of hearing. Probe 

 it with a needle. 



(19) Study the eye. Note the stalk on which the eye is 

 located. Is it segmented? Is it hinged at the base? Is 

 the eyestalk like a swimmeret? Remove the outer covering 

 of the eye, scrape the inner dark part away and mount the 

 outer part under a cover glass. Examine under microscope. 

 Draw. 



(20) After removing all the appendages from the ceph- 

 alothorax and cleaning the framework, look again carefully 

 for traces of rings. If these can be found, study them care- 

 fully at the point of fusion and make a drawing of one side. 

 How does this framework differ from the carapace? From 

 the soft fleshy part of the abdomen? How many rings in 

 the entire specimen? Is there a pair of appendages to each 

 ring? 



(21) Study carefully the carapace. Do you find a mem- 

 brane under the hard crust? Is the carapace homologous 

 with the pleurites of the abdomen ? Put a piece of it in boil- 

 ing water, note result; put a piece in dilute hydrochloric 

 acid, note result. Explain what happened in each case. Has 

 the carapace any connection with the framework of the 

 thorax proper? What do you think is the origin of the 

 rostrum ? Of the carapace ? 



(22) Make large drawings of dorsal and ventral sides. 

 Internal Structure. — (i) Place the crayfish, ventral 



side down, on a block of wood, or in a dissecting tray, and 

 thrust pins through the telson and through each of the 

 great claws. Remove the carapace and the gills, leaving 

 the framework and appendages. In removing the carapace, 



