LYMN^ID.E OF NORTH AMERICA. 41 



dent with the appearance of a large number of Hydra viridis in the 

 pond. The Hydra disappeared the next season and the next generation 

 of peregra was of the normal form. While this evidence is largely cir- 

 cumstantial, it is clearly evident that the Hydra was the cause of the 

 malformation. The case cited by Sykes may be of the same character. 

 It is recorded by K. Hurlstom Jones 1 and by Arthur G. Stubbs 2 

 that Lymncea peregra has been known to leave its shell when diseased. 

 This record requires further confirmation. 



g. PARASITISM. 



Various specimens of Lymnaea serve as host for different species 

 of Trematode worms. The following European examples are cited by 

 Cooke 3 : 



Distoma cndolabrum Duj. finds its first immediate host in Lym- 

 ncea stagnalis and Lymncea ovata, its second in Lymncea stagnalis, or 

 in one of the fresh-water shrimps and attains to sexular maturity in 

 the common frog. Distoma ascidia v. Ben. passes, first through Lym- 

 ncea stagnalis or Planorbis corneus; secondly, through certain flies and 

 gnats (Ephemera, Perla, Chironomus) and finally matures in certain 

 species of bats. Cooke 4 records the following interesting notes on 

 this subject: 



"The common liver-fluke, which in the winter of 1879-1880 cost 

 Great Britain the lives of no less than three million sheep, is perhaps 

 the best known of these remarkable parasitic forms of life. Its his- 

 tory shows us, in one important particular, how essential it is for the 

 creature to meet, at certain stages of its existence, with the exact 

 host to which it is accustomed. Unless the newly-hatched embryo 

 finds a Limncea truncatula within about eight hours it becomes ex- 

 hausted, sinks and dies. It has been tried with all the other common 

 pond and river Mollusca, with Limncea peregra, palustris, auricularia, 

 stagnalis, with Planorbis marginatus, carinatus, vortex, and spirorbis, 

 with Physa fontinalis, Bithynia tentaculata, Paludina, Vivipara, as 

 well as with Succinea putris, Limax agrestis and maximus, Avion atev 

 and hortensis. Xot one of them would it touch, except occasionally 

 very young specimens of Limncea peregra, and in these its development 

 was arrested at an early stage. But on touching a Limncea truncatula 

 the embryo seems to know at once that it has got what it wants, and 

 sets to work immediately to bore its way into the tissue of its invol- 

 untary host, making by preference for the branchial chamber; those 



iJourn. Conch., IX, p. 164. 

 2 1. c. p. 112. 

 3 Mollusca, p. 61. 

 4 op. cit., pp. 61, 62. 



