MARINE BIOLOGICAL STATION ON PUFFIN ISLAND. 51 
glass jar of fresh water. In this they remained day after 
day with the operculum or lid which closes the mouth of 
the shell tightly shut. At the end of the second day, I 
took one of the specimens out of the water, and on opening 
the shell found that the animal was alive and active inside. 
On the fourth day two specimens died, on the sixth day 
four more, and by the end of the eighth day all of them 
were dead. Whether this death was due merely to the 
prolonged immersion in the fresh water, or may have 
been caused by the water becoming slightly impure, was 
uncertain, so the experiment has been repeated several 
times since (see below, jar B). 
It is easy to tell whether the Littorinas are alive or 
dead, as, so long as a specimen is alive, it remains 
tightly shut up in its shell, while whenever it dies the 
operculum opens and a part of the ‘‘foot”’ of the animal 
protrudes in the form of a white mass, which rapidly 
begins to decompose. 
I next collected from the rocks a fresh set of specimens, 
which were placed as follows :— 
(A) Ten specimens in a jar of clear sea-water, under 
muslin (see below, fig. 3). 
(B) Ten specimens in a jar of fresh water. 
(C) Ten specimens in an empty dry jar (¢.e. in air). 
(D) Twenty specimens in a slate and glass aquarium, 
half full of sea-water and open at the top. 
The jar A (see fig. 3) was so arranged as to have a piece 
of coarse muslin (m) spread over a hoop just below the 
surface of the sea-water, the object being to allow the air 
to have free access while preventing the molluscs from 
coming to the surface of the water. 
These four sets of specimens were examined every twelve 
hours for three days, and their positions and apparent 
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