The Young of the Crayfishes Astacus and 



Cambarus. 



By E. A. Andrews. 



INTRODUCTION. 



Tlie object of the present paper is to figure and describe the external 

 forms and the appendages of the early larval stages of the crayfishes Astacus 

 and Cambarus and to illustrate the details of the connections that exist be- 

 tween these larvje and the mother. 



It is well known that the genus Cambarus is found only in North America 

 and here only to the east of the Rocky Mouutains, while the genus Astacus is 

 the only one found in Europe and Asia and in America is found almost ex- 

 clusively west of the Rocky Mountains. It is thus natural that the history of 

 scientific knowledge of crayfishes has been first the study of Astacus in Europe 

 and later the study of Cambarus in the eastern United States. The Astacus of 

 the Pacific States remains less well known. 



Despite all the work that has been done upon these common animals, 

 several ])arts of their life histories have received scant attention. The geo- 

 graphical distribution and systematic description have been studied in detail by 

 liageu, Faxon, Huxley, Ortmann, and others; the embryology minutely ob- 

 served by Rathke and by Reichenbach ; and the general knowledge of crayfish 

 natural history added to again and again since the days of Roesel von Rosen- 

 hof. Yet little attention has been given to the study of the young crayfish after 

 it leaves the egg; a comparative neglect that tiatin-ally arose from the centering 

 of scientific interest upon the larval changes of marine crustacea in which re- 

 markable metamorphoses occur. 



When these metamorphoses were estal)lis]i('(l I)y Vaughn Thompson and 

 others it was already known from the work of Rathke that the crayfish hatches 

 from the egg in essentially the adult shape and thus passes through no series 

 of metamorphoses. Interest in the crayfish young was then restricted to the fact 

 that it was exceptional in having no metamorphosis. In the ]n-eoccupation of 

 students of crustacean life histoi'ies in study of metamoi'phoses the young of 

 the crayfish were left without any illustrations excepting only those given by 

 Rathke to show the condition of the embryo when nearly ready to hatch and the 

 two wood cuts given 1)y ITuxlcv. Tb(\'<(> latter illustrations were evidently 



