OF NORTH AMERICA. 47 



its fringe of cilia and its locomotor function (see plate II, figures C, 

 D.i). 



The young Lymnsea, just before it leaves the shell, is an interest- 

 ing object, especially when viewed with a lower power (about J^ inch 

 objective) by the aid of which its spiral journeyings inside the egg 

 may be observed (plate II, figure B)< The shells at this time consist 

 of a trifle less than two whorls, which are flat and helix-like, quite 

 unlike the long-spired shell of the mature animal. 2 The eggs of Lym- 

 nsea are easily obtained and the development of the embryo may be 

 plainly seen and studied with ordinary powers of a compound micro- 

 scope. Much good work may be done by anyone who will compare the 

 development of some of the common species. 



The effect of different colored lights and also the different den- 

 sities of light on the developing eggs of Lymngea has been interest- 

 ingly shown in a series of experiments by E. Young 3 who records 

 the following variations in the time of hatching: In violet light, 17 

 days; in blue light, 19 days; in yellow light, 25 days; in white light, 

 27 days; in red light, 36 days; and in darkness, 33 days. 



k. EXPERIMENTS. 



The genus Lymnsea has been frequently used in conducting ex- 

 periments relative to the animal's power of resisting changing condi- 

 tions of the environment. Whitfield's experiments on Bulimnea 

 megasoma* are well known. Specimens of this large Lymnsea were 

 confined in an aquarium of small size and were bred to the fourth 

 generation. Each succeeding generation was smaller than the one 

 preceding, the last or fourth generation producing a shell about half 

 the size of the first generation, with an elongated spire and generally 

 narrow form, quite unlike the robust form of typical megasoma. Whit- 

 field concluded that the volume of water bore a very close relation to 

 the size and perfection of the shell. Certain interesting changes were 

 also noted in the organs of the animal, such as the loss of the male 

 sexual organs and a notable reduction in the size of the digestive 

 glands. This degeneration is thought by Dall to be due rather to a 

 concentration of salts, as the evaporated water was replenished with- 

 out emptying the residual supply, causing an excess of salts to accumu- 

 late. 



*See Lankester, Quart. Journ. Micr. Soc., Vol. XIV, p. 365, 1874, for an 

 extended account of the development" o"f Lymnaea. 



2 See an interesting account on this subject by Jabez Hogg, in Trana, 

 Micr. Soc. London, II, p. 91, 1854. 



8 Arch. Zool. Exper., VII, pp. 273, 274. 



4 Bull. Amer. Mus. Nat. Hist., I, p. 29, 1881. 



