PHYSA. 51 



horn-colour, with a faint greenish or yellowish tint, with 

 faintish strise in the line of growth, occasionally marked with 

 rather stronger ridges in the same direction, and very in- 

 distinctly striate spirally ; whorls \-^^ tumid, body whorl much 

 larger than the rest of the shell ; spire short, apex obtuse \ 

 suture deepish ; mouth oblong, large, somewhat contracted, and 

 acutely angular above, rounded below ; outer lip very thin ; 

 inner lip widely spread over the base of the penultimate whorl ; 

 pillar with a slender fold. 



Inhabits slow rivers, brooks, ponds, and ditches on 

 aquatic plants, particularly watercress, all over Great 

 Britain, and is common. 



This is a more than usually active mollusc ; it 

 jerks itself from place to place along the water in a 

 manner which it is amusing to witness. Gwyn Jeffreys 

 says : " The jerking motion which this animal has 

 is said to be owing to its being infested by a small 

 kind of parasitic worm, which causes it some un- 

 easiness. I should rather be inclined to attribute 

 this motion to the length and narrowness of the foot, 

 which has to support a comparatively bulky shell." 

 — B.C.y vol. i. p. 99. I have before me now a glass 

 vase, in which are several kinds of aquatic molluscs, 

 which I have placed there for the purpose of ob- 

 serving their habits. They are all more or less 

 infested by parasites, which do not appear to cause 

 them much, if any, annoyance. Among them there 

 are several specimens of Planorbis corneus, whose 

 bodies, more especially about the head, are literally 

 swarming with vermiform parasites, that wriggle 

 about in a manner one would suppose to be anything 

 but comforting to their victims, which seem, however, 

 totally unconcerned, and slowly creep round the 



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