GERM-CELL HISTORY IN THE BROOK LAMPREY 7 



figure 2. The number of specimens measured in this case is 

 130. It will be seen that this curve has four distinct humps. 

 The larvae, which in the first curve were represented by the fifth 

 hump, have Supposedly metamorphosed and are at this time of 

 the year in the adult form. The other humps of the curve 

 represent larvae about one, two, three, and four years old. 



Two other curves are shown, each of which is constructed 

 from combined collections of three successive months. The 

 curve for April, May, and June (fig. 3) has the peaks of the 

 humps at 3, 9, 12, and about 14 cm., respectively. The humps 

 represent larvae one, two, three, and four years old. At this 

 time of the year adults are found and also larvae that have just 

 hatched, but these are not included in the graphs. The other 

 curve from the months of July, August, and September shows 

 practically the same thing. It appears from these curves that 

 Entosphenus wilder! probably attains an age of five years before 

 it is sexually mature. This does not exclude the possibility that 

 individuals may reach their full size in four years. 



The rate of growth varies greatly, as shown in figures 1 to 4, 

 where individuals grouped in each hump of the curves are pre- 

 sumably of the same age. The twenty-six larvae represented 

 by the first hump in figure 1 range in length from 15 mm. to 

 32.5 mm. and have an average length of about 24 mm. As 

 shown by the curves, the same variation in the rate of growth 

 exists among older larvae. This probably is due largely to dif- 

 ferences in the food supply in different parts of the stream and 

 to the inactivity of the larvae, which keeps them from seeking 

 out the optimum environmental conditions. It was found, for 

 instance, when the larvae were kept under observation in the 

 laboratory, that they remained in their burrows for days at a 

 time, even though they were kept in a small dish without running 

 water. It was only when the water became warm and stale 

 that they came out. In the stream the water is cool and well 

 aerated, so that it is doubtful if the larvae ever come out unless 

 disturbed by torrents after heavy rains and during spring thaws. 



The time of metamorphosis probably varies somewhat in dif- 

 ferent species and also in different localities. Balfour ('81) says 



