LYMNZID OF NORTH AMERICA. 4] 
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' and by Arthur G. Stubbs? 
that Lymnea peregra has been known to leave its shell when diseased. 
This record requires further confirmation. 
g. PARASITISM. 
Various specimens of Lymnza serve as host for different species 
of Trematode worms. The following European examples are cited by 
Cooke? : 
Distoma endolabrum Duj. finds its first immediate host in Lym- 
nea stagnalis and Lymnea ovata, its second in Lymnea 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- 
nea stagnalis or Planorbis corneus; secondly, through certain flies and 
gnats (Ephemera, Perla, Chironomus) and finally matures in certain 
species of bats. Cooke 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 Limnea 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 Limnea peregra, palustris, auricularia, 
stagnalis, with Planorbis marginatus, carinatus, vortex, and spirorbis, 
with Physa fontinalis, Bithynia tentaculata, Paludina, V ivipara, as 
well as with Succinea putris, Limax agrestis and maximus, Arion ater 
and hortensis. Not one of them would it touch, except occasionally 
very young specimens of Limnea peregra, and in these its development 
was arrested at an early stage. But on touching a Limnea 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 
Journ. Conch., IX, p. 164. 
‘1. ¢.. p. 112 
8Mollusca, Dp. 61. 
40p. ‘cit., ‘pp. 61, 62) 
