VARIATION IN FORM. 
65 
Fig. 139. —An Univalve with 
Gibbously Inflated body whorl. 
Lifuncea stagnalis X}-..')^ 
Pond, Osmondthorpe, Leeds, 
Collected by Mr. H.Crowther, 
F.R.M.S. 
Tin's transverse bulging of the wliorls during the course of growth 
is occasionally observed in a pronounced form in many species, 
especially in Limncva i^tacjiKilh, and would appear to he a very general 
outward biological expression of a number 
of abnormal conditions, as well as of a 
period of superabundance of nourishing and 
palatable food. In the autumn of 1894 
all the specimens of Limncca stagnalis 
found in a small pond at Osmondthorpe, 
near Leeds, exhibited this remarkable in- 
flation upon the body whorl, but those 
gathered during 1895 in the same place did 
not show the peculiarity, so that the cause 
of the expansion, whatever may be its 
nature, was evidently an evanescent one. 
The pond has a rich and varied vegetation 
and contains many forms of animal life, and it may be remarked that 
leeches, which Ilerr J. Ilazay states are 
sometimes the cause of whorl expansion, 
were numerous in it. The very interesting 
specimen of L. stagnalis var. hodamica, 
which I owe to the kindness of Mr. Brockton 
Tomlin, also shows this peculiarity in a very 
striking manner. 
Identical inflated growths are occasionally 
developed by specimens in confinement, 
and Mrs. Skilton has kindly furnished me 
with a characteristic sketch of a specimen 
of Limnwoj stagnalis reared in her aqua- 
rium, which distinctly shows this remarkable whorl distension. 
The Pelecypods, more especially Anodonta cygnea, are liable to a 
similar inflation, though less markedly so than the Univalves ; this 
gibbous swelling, when it does occur, is naturally of a concentric 
character, in harmony with the lines of growth or increase. 
The various Labiate or lipped forms have had several tlieories 
advanced to account for their peculiarities. In some cases the sug- 
gested explanation of the origin of the gibbous varieties may to some 
extent apply to this deviation also, and if so the determining cirenm- 
Fig. 140. — An Univalve with 
Gibbously Inflated body whorl 
(dorsal aspect). 
L. stagnalisv, hodaviica Miller, 
Boden See. 
Collected by Herr Schenck. 
E 
