66 THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBARUS 



when sixteen months old. But here again the individual differences are very 

 great. 



In the second winter there was probably no growth; two crayfish 75 and 

 80 mm. long in October, 1904, were the same length in May, 1905, and, in gen- 

 eral, moulting during the early spring was rarely observed. 



The only observation upon the growth of crayfish reared from the egg 

 and kept during the third summer was the following: two individuals 70 and 

 79 mm. long when twenty-four months old left one survivor 90 mm. long in 

 October, 1903, when 28 months old. It had thus increased either 11 or else 

 20 mm. in the third summer, that is, it added only 14 per cent or 20 per cent 

 to its length. 



Probably then a crayfish may grow in the third summer from a length of 

 three inches to be somewhat short of four inches. 



The one young crayfish kept during its third winter did not change but 

 measured 90 mm. in October, 1903, and the same again in May, 1904. 



In its fourth summer this crayfish grew to a length of 98 mm. by October 

 but died then when three years and four months old, having grown but 8 mm. 

 or barely 9 per cent in this summer. This may indicate a great diminution in 

 rapidity of growth as the maximum size is approached so that the crayfish one 

 hundred and twenty millimeters long that are taken in the Potomac may well 

 be six or seven years old; but from one specimen we cannot say that the rate 

 is naturally lessened in the fourth year and a possible yearly increment of one 

 inch would make the large crayfish of five inches only four years old. 



The age of Cambarus affinis would seem to be roughly determinable from 

 the formula A = L V, where A = the age in years ending in May, L = the 

 length in inches and V= one, in conditions of maximum favorableness, but in 

 unfavorable conditions V may become zero and in very old large crayfish V 

 may be a negative quantity. 



Soubeiran ('65) gave the length of Astacus in France as 50 mm., 70 mm., 

 90 mm., 110 mm., 125 mm., in the first to fifth years inclusive, that is with an 

 annual increment of 20 mm., which, however, became less in the fifth year, so 

 that he could not tell the age of crayfish 160 mm. long nor of any very old ones 

 190 mm. long. 



Compared with this Astacus, C. affinis starting as a much smaller egg 

 caught up in size in the first summer and for two years showed just about that 

 same increment of 20 mm. annually, but then probably began to grow with more 

 diminished speed than did the French Astacus. 



The sexes of these young crayfish are early determined and in the third 

 larval stage the female shows the beginning of the annulus and the male the 

 external openings of the deferent ducts, as elsewhere described (Andrews, :06). 

 But the difference between the appendages of the first abdominal somite in 



