4 VARIATION AND CORRELATION IN THE CRAYFISH. 



CHARACTERS STUDIED AND METHODS USED. 



In choosing characters for measurement two somewhat conflicting 

 things had to be kept in mind. In the first place, to get adequate evi- 

 dence on the problems attacked it was necessary that several characters 

 should be taken in each of several members of a homologous series of 

 organs. In the second place, practical considerations with reference to 

 the time which could be given to the work demanded that the total number 

 of measurements to be taken on each individual should not be too great. 

 After careful consideration of the matter it was decided that the joints of 

 the legs offered on the whole the best chance of getting light on the 

 relation of variation to differentiation and homology. The legs are more 

 satisfactory appendages to deal with than the maxillipeds, because of their 

 greater size. Further, the morphological specialization of parts connected 

 with the performance of special functions does not extend so far in the 

 case of the legs. 



Fig. 1. — Outline of leg of crayfish bearing great chela. 



It did not appear necessary for our purposes to take measurements of 

 the same characters on both sides of the body. Consequently all measure- 

 ments of bilateral characters were taken on the organs of the right side 

 of the animal only. It will be understood without further reference to 

 the matter that throughout the paper all statements regarding the joints 

 of the legs are based on data from the appendages on the right side of 

 the body. On each of the first three legs (i. e., the cheliped and the 

 first two walking legs) the length of each of the three distal joints was 

 measured. These distal joints were taken rather than more proximal 

 ones, because of the greater degree of differentiation which they present. 

 In addition to the measurements on the legs, the length of the cephalo- 

 thorax and the breadth of the head were recorded. The following 

 detailed statements in connection with fig. 1 will make plain the exact 

 character of the measurements taken. 



For verbal convenience we shall throughout the paper use the follow- 

 ing conventions in referring to the different legs: The leg bearing the 

 great chela will be designated as "leg i ;" the first walking leg as "leg ii ;" 

 and the second walking leg as "leg iii." 



