CRAWFISH 71 



III. HOMOLOGIES OF THE APPENDAGES 



In this entire section where drawings are asked for they are 

 to be ventral views of the appendages from the left side of the 

 body. 



1. Study one appendage of the third abdominal segment. 

 Distinguish the stem or protopodite, composed of a basal short 

 segment the coxopodite and a long segment the basipoditc. Of 

 the two branches given off from the end of the protopodite the 

 outer one is called the exopodite, while the one nearer the median 

 line of the body is called the endopodite. Draw X4 and label 

 all of the parts. 



2. Such a two branched appendage is called a biramous ap- 

 pendage, and constitutes the general plan upon which all of the 

 appendages of the crawfish are built. In the following study each 

 appendage is to be studied in order to determine the modifica- 

 tions which have arisen in the various parts of biramous appen- 

 dages. 



3. The study of homology usually involves a comparison 

 of organs or structures found in two different kinds of organisms. 

 When repeated parts upon the same individual show the same 

 fundamental plan of structure the term serial homology is used. 



4. The terminal part of the abdomen is the tclson. The 

 telson together with the appendage of the sixth abdominal seg- 

 ment form the tail fin. Draw the entire tail fin X3, ventral view, 

 labelling all parts and being especially sure to show what parts 

 of the sixth abdominal appendage correspond to the parts worked 

 out for the third appendage under section T. 



5. Remove carapace from the animal's left side exposing 

 the gill chamber. 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 ad- 

 vantage when under water. 



