130 GENERAL ZOOLOGY 



By doubling underneath it is capable of striking hard blows 

 against the water and forcing the crayfish suddenly backward. 

 A basal stem with two branches is to be clearly seen in all 

 the abdominal appendages except the first pair in the female. 



Homology and Analogy. The fact that a stem and two 

 branches are apparent in several appendages which have been 

 mentioned, is important enough to deserve further discussion. 

 The simplest condition of the branched appendage is seen in 

 the third, fourth, and fifth abdominal appendages (Fig. 68, XVI). 

 If the branches are spread slightly, the form resembles the 

 capital letter Y. Taking a swimmeret as a model, the stem, 

 which is termed the protopodite (Fig. 68, 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 nearest 

 the median line of the body is the endopodite (Fig. 68, XVI, 2); 

 the outer is the exopodite (Fig. 68, 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 endopodites and 

 the exopodites. 



The legs seem to be formed on a different plan, but we 

 know from the embryology (the science of the early stages 

 of development) of the crayfish that while the egg is still 

 unhatched the embryo has legs that have two branches (com- 

 pare larval lobster, Fig. 70), and that the outer and smaller 

 one disappears before the embryo hatches. The first and 

 second 'maxillae have the parts of a typical crayfish append- 

 age, divided as shown in Fig. 68, IV, V. The crushing part 

 of the mandible is thought to represent the coxopodite and 

 basipodite combined. The little palpus is the endopodite; 

 the exopodite is missing in the adult. 



In this brief description of the appendages of the crayfish 

 two very important facts in morphology (the science of form) 

 have been suggested. One, the inherent similarity of structure 



