22 ASTACIDJE. 



all the young male specimens of Cambarus, without exception, pertain 

 to the second form. 15ut there are also not a few large males belonging 

 to the second form, which have retained the articulation and the shape 

 of the tip of the first abdominal legs proper to the young specimens. 

 The articulation is sometimes entirely or partially gone. Among nearly 

 fifty fall-grown males of Cambarus aeutus, about twenty-five belong to 

 the second form ; while among fifty young males, nearly a dozen have, 

 for the most part or entirely, lost the articulation. Tab. II. Figs. 1 1 1'. 

 113. 



The discovery that every species of Cambarus possesses two different 

 forms of males was made by Professor L. Agassiz,* and kindly communi- 

 cated to me. 



Dimorpkorism, or perhaps a Sterile Form. — The existence of a second 

 form of the male, if it w'ere no more than a passage or metamorphotic 

 form, would not be extraordinary. But the great number of full-grown 

 second-form specimens in every species, which are often even larger 

 than the first-form males, seems to prove that they are individuals 

 which have remained in a sexual stage that does not agree with then- 

 corporal development, — in short, that they are perhaps sterile. 



The objection that these second-form males may be individuals 

 shortly before or shortly after the casting of the skin I can surely 

 refute, as I have seen many specimens at this stage of growth ; the 

 Museum collections exhibiting the animal in all the different phases of 

 its existence. 



Another objection, that the males of the second form, or perhaps 

 those of the first form, are abnormally developed individuals, is refuted 

 by the great number of the two forms existing and living together. 



The conjecture, on the other hand, that the second-form males may 

 lie sterile, is really supported by the anatomical examination of the two 

 forms in the principal groups of Cambarus. 



Infernal Sexual Parts of Cambarus dlale. Of C aeutus. Tab. II. Figs. 

 120-123. — In two full-grown males of Cambarus aeutus, first and 

 second form, both four inches long, the testicles are trilolate. as in 

 Astacus fluviatiUs, but much smaller, the vasa deferentia shorter. In 

 the first-form males the two superior lobes are pyriform, truncated 

 above, united below in a membraneous hole a little shorter than the 

 testicles, and connected with the third inferior lobe. AVhere the two 

 superior are united, the vas deferens begins on each side, being vermi- 

 form and shorter than the body. The testicles in a Cambarus aeutus, 

 four inches in length, are but 0.3 inch long ; while in an Astacus Jiu- 

 viatiUs, two inches in length, they are 0.6 inch long, thus much larger, 



* Intending manv years ago to describe the North American Astacidse, la- made a close inspec- 

 tion of the material lie had collected for that purpose. Observing the different shape and organi- 

 zation of the first pair of abdominal legs, he was led to this important discovery. 



