26 G. C. Embody. 



schedules of generations, number of broods in each, and the number of 

 eggs produced by each brood, have been made. 



E. gracilis. 



In the case of this species, a computation of the yearly pro- 

 ductiveness is a simple matter for the reason that the young grow so 

 slowly a second generation is not produced until the second year. 



If a single pair breed every 15,5 days during a total period of 

 299 days, they will breed approximately 19 times. The average number 

 of eggs produced at any one time is 45 (table IV). Hence the total 

 number produced during the period of 299 days is 855. 



Schedule I. G. limnaeus. 

 One pair produces an average of 25 eggs, 7 times in 266 days Total 175 



Generation I. 

 Brood 1. 12prs.*)ave., 16 eggs') each on 128*11'') dy. and 3 times thereafter 768 



Generation 11. 

 Brood 1. 8 prs. ave. , 8^) eggs on 255 1^^ dy. and times thereafter 64 



Grand total 1619 



*) The weak point in these calculations lies in the assumption that on an 

 average the number of males in any brood is equal to the number of females. There 

 were no characters upon which the writer could rely for separating young males 

 from females. Hence no accurate knowledge is at hand for determining the nume- 

 rical ratio of their occurrence. The assumption is based solely upon observations of 

 a single brood of eleven Hyalellas of which five young subsequently proved their 

 sex by mating and laying eggs. 



') This average is secured in the following manner: — The individuals of brood 1 

 will become sexually mature at the age of 93 daj^s , approximately 9 mm , long (see 

 curves p. 24) and the females will lay 8 eggs (table IV). They will breed 4 times 

 during the period of 266 days. At the fourth breeding date they will be 107 days 

 old (3 X 38 + 93), 7,5 mm, long (p. 24) and will lay about 24 eggs (table IV). The 

 mean between 8 and 24 is 16 and may be taken as the average number of eggs 

 produced at any one breeding. 



') The eggs producing these young required 35 days for development and the 

 young matured 93 days later; hence 128 represents the day of the first and only 

 oviposition. 



*) These pairs will breed only once, at age of 93 days and will therefore pro- 

 duce but 8 eggs, (table IV). 



^) The young resulting from broods 2, 3 and 4 will not mature within the limit 

 of 266 days. 



