SEASONAL INFESTATION WITH LARVAL NEMATODES. 497 



fluctuations in the total percentage of infestation; the relative 

 percentage of mature larvae to total infestation is shown by the 

 dotted line. It is unfortunate that the May collection was not 

 tested for emergence; but from the available data it is seen that 

 the occurrence of mature larvae continues high from August to 

 December, and then apparently is lower until late July and 

 August, at which time it approximates its former values. The 

 high percentages of emerged C. lintoni in July and August, 1925, 

 are supported by the fact that during these months most of the 

 sporocysts from crushed snails contained fully formed larvae. 

 Plotting of the temperature readings taken at the Fish Com- 

 mission Wharf, 1 probably only in a general way comparable to 

 that at Quamquisset Harbor, seems to show a relation between 

 maturity and temperature; the maturity graph apparently lags 

 behind that of temperature, remaining high while the tempera- 

 ture is dropping in November and December, and low for a 

 considerable period after the temperature has increased in the 

 spring and early summer months. Other factors which have 

 already been discussed as affecting total infestation might also 

 affect the percentage of mature cercariaea. 



It is interesting to note that there is a close general similarity 

 between the graphs shown by Sewell (1922: Chart i) for two 

 species of fresh water snails from India and the total infestation 

 graph for Nassa obsolete in the present study. The maxima for 

 Sewell's examination of Melanoides tuberculatus from the Museum 

 tank, fall in July and December, as they do for Nassa obsoleta, 

 and the partial graph for M. tuberculatus from the Zoological 

 Gardens follows the same general trend, with summer maximum 

 in August rather than in July. Sewell's two graphs are based 

 on 139 and 53 mollusk individuals respectively, while in the 

 present study 8,452 large, and 423 small, specimens of Nassa 

 obsoleta were examined. 



i Temperature data were secured from the records of the Bureau of Fisheries for 

 the ist and isth of each month of the period from August, 1924, to August. 1925 

 the figures are the mean of three daily readings taken at the U. S. Fisheries Station 

 at 8 A.M., 12 M., and 4 P.M. The highest is 71 F. for August i, 1924. and the 

 lowest 30 F., just six months later, February i, 1925- Thus there was at least an 

 annual range of 41 Fahrenheit. 



