This globular object, the size of a small millet
seed, is a hard, bony vesicle which floats in
water. ‘The presence of this vesicle inside a
gull is certain evidence that the bird has fed on
a member of the herring family during the day
—in this case a sprat.
My attention was first drawn to these bony
’ vesicles in the following manner: My procedure
when I examine the contents of a gull is to
remove the entire alimentary tract, then open
the gullet and gizzard, and pick out the larger
objects; the remainder of the tract is then cut
open, and the whole washed out in a basin of
water, filtered, and examined, if necessary,
under the microscope. While I was examining
the contents of kittiwakes which had been
following up shoals of sprats off the East Coast,
I found these hollow globules in large numbers.
Next I boiled the skulls of sprats, and rubbed
the softened bones between the finger and
thumb, and obtained a pair of these globules
with each skull. This is exactly what happens
in the gizzard of the gull; the bones are softened
by the gastric juices, and in the gizzard the softer


110 ANIMAL LIFE UNDER WATER


bones are triturated away, while the denser bone
round an air space on each side of the skull
remains.
