MONSTROSITIES — POLYSTOMATISM. 
119 
Fig. 2G7. — Claitsilia bidcntata 
(Strom) X 2, 
Luton, Ijedfordshire, 
Collected by Mr. J. Saunders, 
Showing the two distinct 
mouths of the shell. 
PoLYSTOMATiSM (ttoAl's many, (TTOfia mouth) is a term denoting the 
presence of more than one aperture to a shell, Rud is most liable to 
occur amongst the individuals of those species with greatly contracted 
apertures, especially those forming the genus Clausilia, which are, 
under certain circumstances, liable to form an additional or second 
aperture. These dual-mouthed shells might possibly be regarded by 
novices as the joint production of two individuals united together, 
but with the head and neighbouring parts distinct and separate — a 
kind of Siamese twins — but this second aperture is probably formed 
on account of some obstacle or hard particle 
becoming fixed, temporarily or permanently, 
in the normal one, thereby deranging or 
obstructing the action of the clausium, and 
preventing the egress of the animal, which 
is therefore compelled to form a new outlet 
or perish confined within its shell. This 
new outlet is made by piercing the shell- 
wall, usually about half-a-whorl distant from 
the original one, and thus facing in the opposite direction, and is 
formed e.xactly in the normal way. Mr. P. B. Ma.son mentions specimens 
of Cldasilid with as many as four distinct mouths to the shell. 
Although the wide-mouthed shells do not appear to be so liable 
to this peculiarity as the ClaudVm, which have a very contracted 
and dentate aperture, yet I have a very interesting distomate speci- 
men of Limnwd auricularia, kindly given 
me by Mr. H. Wallis Kew, and found by him 
at High Bead), Epping Forest, in 1894. This 
shell, from some inexplicable cause, has dis- 
continued the use of the normal aperture, 
and pierced the outer shell-wall or perhaps 
availed itself of an accidental fracture of the 
body-whorl, about a quarter of a volution 
from the outer lip, and there constructed a 
new and more contracted opening which, 
despite its comparatively restricted size, appears to have been used as 
the means of exit. This new growth, forming the protruded new out- 
let, would appear to have been formed after the atrophy of the glands 
of the collar, as it is deficient of epidermis, and is apparently formed 
by the general surface of the mantle. 
Fig. 268 . — Lbunwa auricu- 
laria (L.), 
High Beach, Epping, 
Collected by Mr. H. Wallis 
Kew, F.Z.S., 
Showing two distinct and 
separate apertures to the shell. 
