72 LABORATORY DIRECTIONS 



6. The appendages directly associated \vith 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 

 obscure instances the information about the 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 maxttlipeds, two smaller maxillae, and an ex- 

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

 illae there is a pair of very small structures, the paragnatha. 

 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 crawfish. The following table is prepared as a 

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

 cates that the structure is present, a zero that it is wanting. 



segment appendage exopodite endopodite protopodite epipodite gills 



prostomium antennules o -f- + o o 



I. antennae + + + 



II. mandibles o + H~ 



III. first maxillae o -f- + 



IV. second maxillae -f + + + 



9. On your note paper prepare a table for the thorax and 

 abdomen, using the same column headings as above. Data for 

 filling out this table will be secured in the course of the study 

 and should be' incorporated into the table only after the study 

 has been made as directed and after drawings that are asked for 



