150 THE CRAYFISH 



in a similar position for comparison or drawing in accordance with 

 the instructions in the paragraphs that follow. Such orientation is 

 important for correct understanding of the homologies between the 

 various appendages. It is also important that the parts of each ap- 

 pendage drawn be completely labeled and that the smaller ones be 

 drawn on a generous scale. If desired, the appendages may be prop- 

 erly oriented and glued to cardboard instead of being drawn. 



(b) There are nineteen pairs of appendages. Beginning with the 

 abdomen, count the number of pairs in this region and compare with 

 the number of somites. The most posterior abdominal appendages 

 are the uropods, or tail-feet; the others are the swimmerets. Note 

 again the differences in the two anterior pairs of abdominal ap- 

 pendages in the sexes. Remove the right swimmeret of the fourth 

 abdominal somite by cutting close to the body. A basal piece, the 

 protopodite, bears two terminal pieces, an inner endopodite and an 

 outer exopodite. No matter how much any of the other appendages 

 may seem to differ from this plan of structure, all can be shown to be 

 derived from such a fundamental type. The only exception is found 

 in the case of the antennules, whose homologies are uncertain. Draw 

 the above appendage with the end of attachment upward, the exopodite 

 to the right, and the endopodite to the left. Use this same orientation 

 in all your drawings of other appendages. Adjust scale of this draw- 

 ing and of subsequent drawings of appendages to size of your speci- 

 men. It is desirable to have all these drawings on the same scale. 



(c) Remove and draw the uropod of the right side. Orient and 

 label as above. 



(d) The thorax has eight pairs of appendages as follows: four 

 pairs of walking legs, or pereiopods; the great claws, or chelipeds; and 

 three other pairs farther forward which will be examined presently. 

 Remove the right fourth pereiopod and the right cheliped, being sure 

 to get all the parts of which each is composed. In the pereiopod the 

 two proximal parts represent a divided protopodite, while the remain- 

 ing five are divisions of the endopodite. In the embryo an exopodite is 

 present (c/. Fig. 67). The great claws resemble the two anterior pairs 

 of pereiopods save for the union of two of the divisions. Can you 

 find where this has occurred? Note the modification of the distal end 

 of the appendage. Draw this pereiopod in the same orientation as 

 abdominal appendages, and show by a dotted outline the position the 

 exopodite would have if present. 



(e) Anterior to the great claws are three pairs of appendages, 

 known as maxillipeds, or jaw-feet. The most posterior pair, the third 

 maxillipeds, are large and easily recognizable. Before removal, the 



