685 



spermatozoa are mature and of course, smaller still after the testis has 

 been evacuated. 



So far as I have been able to observe, the testes of first- and second- 

 form males of the same size, taken at the same season of the year, are 

 equally developed. It being impossible to tell from the testes alone 

 whether the animal was first- or second-form, 



Faxon" observed that the so-called first- and second-form males 

 were only alternating stages in the life of the individual. First-form 

 males of C. rusticus Girard, kept in his laboratory copulated freely 

 and soon after exuviated. Later examination showed that the casts 

 were first-form while the animals themselves were second-form. He 

 noticed practically the same thing for C. propinquus Girard. 



I have had the opportunity of making quite extensive field obser- 

 vations on C. immunis Hagen. In the late summer and autumn, the 

 proportion of first-form males gradually increases, there seeming to 

 be, so far as I have yet been able to observe, no definite time at which 

 the exuviation takes place. By late September or early October practi- 

 cally all are in the first-form condition. Copulation probably takes 

 place with this species in the fall. Early in March the males, still first- 

 form, emerge from their burrows and inhabit the stagnant ponds, re- 

 maining in first-form condition until about April 20. At this time al- 

 most all are found to be ready to exuviate, the animal in its new con- 

 dition being second-form. When the old shell is slipped from first-form 

 specimens wich are almost ready to exuviate the appendages of the 

 animal in its new form are seen to be clearly second-form. Some- 

 what less extensive observations lead me to believe that in C. virilis 

 Hagen the process is essentially the same. Thus my somewhat more 

 extended observations confirm Dr. Faxon's important discovery that 

 the first- and second-form are only alternating phases in the life of 

 the same individial. 



Three specimens which have come under my observation are de- 

 serving of especial attention. The first — a specimen of C. immunis — 

 exuviated April 29, 1901, the same day it was taken. An examination 

 of the animal and of the cast plainly showed that the cast was per- 

 fectly-formed second-form as was also the animal in its new condition. 

 In the case of the second individual, I did not secure the cast but took 

 the animal itself while it was yet very soft from exuviation. It was 

 clearly first-form and since, with the exception of the one specimen 

 mentioned above, no second-form males, except those which had re- 

 cently exuviated, were taken this spring, there is a strong probability 



Faxon, On so-called Dimorphism. 



