CEAYFISH 79 



the gill chamber. In the dissection and study which follow, it is 

 imperative for you to know what structures are associated with 

 each of the legs. Begin at the posterior end of the thorax, grasp- 

 ing the last walking leg with the forceps. Move this leg gently 

 about so as to discover all parts attached to it. In like manner 

 manipulate each of the remaining legs without removing or in- 

 juring any of the attached parts. Move the cheliped slightly 

 and note relation to gills. With a pair of strong forceps grasp 

 the cheliped at its attachment to the body and by a firm, steady 

 pull remove the entire appendage, being sure to get all the parts 

 belonging to it. The various parts of all such appendages are 

 studied to best advantage when under water. 



6. The appendages directly associated with the mouth open- 

 ing have become greatly modified in their adaptations to special 

 functions. In these highly modified appendages relative position 

 of the parts is not a safe clue to homology. From the study of 

 these structures in their early development it is usually possible 

 to determine the homologies with certainty. In these and other 

 obsecure instances the information about homologies is given 

 in this outline. 



7. With the cheliped out of the way it "is easier to see the 

 appendages immediately around the mouth. Before removing 

 any other parts study the appendages just in front of the region 

 from which the cheliped was removed. Note that there are three 

 rather conspicuous maxiUipeds, two smaller maxillae, and an ex- 

 tremely hard mafidible. Between the mandibles and the first max- 

 illae there is a pair of very small structures, the jmraynatha, 

 which occur at the sides of the mouth and form the posterior 

 boundary of the mouth. They are outgrowths of the body wall 

 and not true appendages. 



8. A tabulated summary is of interest in showing the re- 

 lationship of the parts in the variously modified biramous ap- 

 pendages of the crayfish. The following table is prepared as a 

 summary of the study of the head appendages. A plus sign 



