CEAYFISH 81 



of the small feeler-like structure attached to its outer margin are 

 protopodite. The two remaining segments of the feeler-like ap- 

 pendage are endopodite. Draw. 



15. Examine the second pair of feelers, the antennae, and 

 compare the parts with those of the third abdominal appendage. 

 Draw left antenna X3. In this, as well as in other parts of the 

 outline, the position of an appendage is determined by the point 

 of its attachment to the body wall, not by the distance the ap- 

 pendage extends beyond the body. 



16. Note the small white elevation of the ventral side of 

 the basal joint of each antenna. The small opening on each of 

 these elevations is the excretory opening. 



17. The first pair of feelers, the antennules, though two 

 branched, are not "biramous." Their location upon the prosto- 

 mium, which is not a true segment, their development, and their 

 structure all indicate that they are not homologous with the other 

 appendages. Note that three joints are interposed between 

 the body and the two branches or flagella of each antennule, 

 while but two joints are found in the protopodite of a true 

 biramous appendage. 



18. While the walking legs of the crayfish are not biram- 

 ous in any stage of their development there are indications that 

 they are biramous appendages which have lost the exopodite. 

 The crayfish is a near relative of the lobster, but the develop- 

 ment of the crayfish has been greatly shortened so that the 

 young, when hatched from the egg, resembles the parent. On the 

 other hand the lobster passes through a distinct series of free- 

 living larval stages before acquiring the general body form of the 

 adult. In these larval stages of the lobster the walking legs are 

 typical biramous appendages made up of protopodite, exopodite, 

 and endopodite. During later development, the exopodite dis- 

 appears, leaving only protopodite and endopodite for each walk- 

 ing leg. These same parts are recognized in the walking legs 

 of the crayfish. 



