SVKES : NOTE ON LIMN^A AURICULARIA. 



35 



or more and cutting the striae of growth at right angles so as to 

 interrupt them. Some specimens show only a line of injury to the 

 striae of growth and no notch, others again have had a notch which 

 has been filled up and the subsequent growth has gradually become 

 of a normal character. Mr. CoUinge found, some years ago, specimens 



of Liiniuea stagtmlis which, from sketches he has sent me, appear to 

 have been affected much in the same way. 



Now how is this to be accounted for? There are too many 

 specimens for the injury to be attributable to accident. Can the 

 deformations then be due to some injury to or disease of the mantle 

 which is common to all ? This seems hardly likely as the deformation 

 seems to be spasmodic, sometimes increasing and again decreasing 

 till the growth is normal, while at other times it increases until a notch 

 is found which remains at the edge of the lip. No attachment of 

 weed will explain these in the same manner as sufficed for some 

 Littorina which I examined recently.* They cannot be due to the 

 pseudo-parasitic Naisox Gordius:\.'~, these take up their position between 

 the neck and mantle and so would not injure the shell-growth. The 

 following extractt is of interest and suggests a clue. " M. Brot has 

 observed, in a lake near Geneva, a deformation affecting nine-tenths 

 of the specimens of this species (Z. peregra) and consisting in a 

 deformation of the base of the columella. This lake contained at the 

 same time an extraordinary abundance of Hydra viridis. A year later 

 not a single specimen of Limnaa was abnormal and since then not 

 a deformed shell has been observed in the lake. At the same time 

 Hydra viridis had totally disappeared. Is then the Hydra the cause 

 of this deformation? One cannot affirm it; nevertheless the coincidence 

 is striking." 



The Hydra theory then seems the most possible though one can- 

 not regard it as proved, and the following may be suggested as the 

 mode of causation. That on the attachment of the Hydra its powers 

 of annoyance cause the sensitive mantle to shrink from the spot while 



* Pro. Dorset N. H. and A. F. Club. xiii. (1892), pp. 191-S, pi. 

 t Pro. Verb. Soc. Mai. Belg. vi. (1887), p. xhiii. 



