104 MESSES. NORMAN AND STEBBING- ON THE 



to different species, which, indeed, he assigns to distinct genera, holding that, while 

 Dohrn's pi. xxvii. figs. 6-18 belong to the Leptochelia dubia of Kroyer, pi. xxvii. 

 fig. 17 (forma altera maris) must be distinguished, and accordingly naming it 

 Heterotanais anomalus. 



The subject is one of great interest, and we trust, at no distant period, will receive 

 full elucidation. 



It has been long known, as observed by Agassiz, Clark, and Hagen, that two 

 distinct forms of the adult male exist in the freshwater Crayfish of the United States 

 belonging the genus Cambarus ; and Dr. Hagen suggested that of these forms the less 

 differentiated, which in many respects much more closely resembled the female, were 

 sterile, while the more highly developed and specialized form was the fertile male. 

 Within the last few months an entirely new light has been thrown on the connection of 

 these two forms by Mr. Walter Faxon \ He has kept in confinement the highly-specialized 

 males together with females, and succeeded in breeding them freely. It will be well now 

 to quote his own words :• — " After pairing, three of the males moulted, and were thrown, 

 while in a soft-shelled state, into alcohol, together with their exuviae. An examination 

 of these specimens now reveals the fact that the soft-shelled specimens are all of the 

 ' second form ' (i. e. that which is less differentiated, and more like the female) ; their 

 exuviee of the ' first form ' (i. e. the highly specialized male). After attaining the ' first 

 form,' and after pairing, the same individual has reverted to the ' second form.' It is 

 now clear that we are not dealing with a case of true dimorphism, such as is well known 

 among insects and plants, but it appears probable that the two forms of the Crayfish 

 are alternating periods in the life of the individual, the ' first form ' being assumed 

 during the pairing-season, the ' second form ' during the intervals between the pairing- 

 seasons. It is to be inferred that before the animal is again capable of reproduction 

 another moult will bring it again into the ' first form.' The fact that large collections 

 made at one time and place often contain only one, or a great preponderance of one, 

 form is now explained." 



Mr. Faxon has also observed this same phenomenon in the case of another species, 

 the Cambarus propinquus, Girard. He remarks, in conclusion, that the " males of extra- 

 ordinary size which I have seen are all of the ' first form.' " Do these very old indi- 

 viduals cease to moult 1 ? Do they become permanently capable of reproduction 1 



Among the Cumacea the males do not attain their most distinctive characters until 

 ready for breeding. While young, and up to a period when, from their size, we might 

 suppose them to be mature, not only in the deficiency of those organs which, directly 

 or indirectly, might be presumed to be connected with the generative functions, do they 

 present features which assimilate to the female, but even in the form and armature of 



1 Faxon, "On the so-called Dimorphism in the Genus Cambarus," American Journal of Science, vol. xxvii., 

 January 1884. 



