150 GENERAL ZOOLOGY 



pairs, called swimmerets, are alike in both sexes. The pair 

 of appendages of the sixth somite are made up of broad, 

 flat branches jointed to a short, thick stem. With the last 

 somite, called the telson (which is without appendages), they 

 form a strong tail fin. By being doubled underneath it is 

 capable of striking hard blows against the water and forcing 

 the crayfish suddenly backward. A basal stem with two 

 branches is seen in all the abdominal appendages except in 

 the first pair of the female. 



Homology. A stem and two branches are apparent in 

 several appendages. This is important enough to deserve 

 further discussion. The simplest condition of the branched 

 appendage is seen in the third, fourth, and fifth abdomi- 

 nal appendages (Fig. 84, XVI). If the branches are spread 

 slightly, the form resembles the capital letter Y. If we 

 take a swimmeret as a model, the stem, which is termed 

 the protopodite (Fig. 84, XVI, 1), is seen to be made up of a 

 short basal segment, the coxopodite, and a long segment, the 

 basipodite. Of the two branches, the one nearer the median 

 line of the body is the endopodite (Fig. 84, XVI, 2) ; the 

 outer is the exopodite (Fig. 84, XVI, 3). Wherever in the 

 series of appendages we find a stem and two branches, 

 the protopodite corresponds to the protopodite of all the 

 other appendages of the series, — and so with the endo- 

 podites and the exopodites. 



All the remaining appendages of the crayfish are con- 

 structed on the same general plan as the swimmerets ; but 

 the original plan, in being modified for different functions, 

 has frequently become obscured. Thus in the walking legs 

 we have no evidence of a two-branched condition. In the 

 very early stages of development the young crayfish before 

 hatching has legs that bear two branches, but one of these 

 disappears entirely before the young is hatched. In the case 

 of the lobster the young animal (Fig. 86) still has the two- 



