73 



questionably shorten the growth period of the Bryozoa. From 

 his researches in Tih'suna- and Liiner-See ca. 2000 m. above 

 sea level Zschokke (1900 p. 117) remarks, that the period of 

 active growth in Alpine lakes can only be estimated at 8 — 10 

 weeks, whereas in low countries at 20 — 22 weeks. The pro- 

 duction of the statoblasts does not take place until ca. 9 — 10° 

 C, i. e. in the latter part of July, and already in Oct. the 

 colonies are dead. During the period Oct. — July only the germs 

 winter in the statoblasts. According to Rink's statement as 

 to the mean temperature of Jacobshavu it appears, that the 

 above mentioned lake has hardly been free from ice for more 

 than three months. "Generally speaking the ice begins to form 

 at the end of Sept., and in the course of Oct. the lakes are 

 quite frozen over; the ice does not wholly vanish until the 

 beginning of July or even later" (Rink I, p. 76). The period of the 

 year when the temperature exceeds ca. 9 — 10° C. is undoubtedly 

 very short. Notwithstanding the severity of the climate the 

 colony might in all probability survive the winter in the case 

 of a favorable locality; in 1899 p. 45 Lampert mentions the 

 Fredericella as being the only Bryozoa positively known to be 

 able to live beneath the ice. 



A few years ago (1904 p. 337) Zykoff was able to report 

 Estheridce as having been found on Kolgujew-Island at ca. 70° 

 N. with an isotherm of ca. 7—8° C. in July. This discovery 

 at one stroke extended the limits of distribution of these or- 

 ganisms from regions bordering on the Mediterranean, from 

 the Hungarian steppes and from the Mongolian as far north 

 as to the arctic zone. The discovery of the Esteridæ along 

 with that of the fresh-water Bryozoa treated of in this paper 

 in the first place goes to show how very little we at present 

 know as to the arctic fresh-water fauna. It has of late be- 

 come customary, from a knowledge of the lowest July tem- 

 perature at which a given organism at the present time is 

 supposed to be able lo live, to draw conclusions as to the 



