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Shelled slugs – Testacella |
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Thursday, 15 November 2007 |
Is it a slug? At the very end of the body it
carries a shell. I have seen this type of slug
only twice, both times on the rough grassy
area of the field near my house.
One normally separates slugs from snails,
in that snails have shells and slugs do not.
But as with all things in nature, the limits are
fuzzier than that. The
division between slugs
and snails, although
convenient, is more a
popular classification
than a scientific one. In
fact, nearly all slugs
have the internal
remnants of a shell,
suggesting that the
ancestor of all of them
was a form of snail. In July, I wrote here that
all slugs have a mantle fold behind the head –
all common slugs but Testacella is an
exception. So not only does this slug have a
shell but it has no mantle. For these reasons
the animal is an oddity.
There are three species of Testacella in
Britain and France. One rarely sees them
because it seems they spend much of their
time underground searching for and eating
earthworms. They are probably often
overlooked as just another slug. They are often
found in the earthworm-rich soil
of gardens.
They have eyes on tentacles just like
garden snails. Many a child delights in
watching the eyes pulled in as a snail touches
an object with its tentacle, and then unroll
again, from the inside out. In the photo you
can see that the right eye is half enfolded.
At the back of the body, emerging from the
front edge of the tiny ‘redundant’ shell, are
two dark grooves or lines. I have no idea what
they are for. Indeed,
perhaps the shell is not
as redundant as one
might think: these
grooves end there,
which could suggest a
sensory function.
The animal can only
breathe through the
skin. There seems to be
no other breathing
surface. This would force it to stay moist at all
times, since a moist surface is needed for gas
exchange.
The Testacella can extend itself to about
8cm and when it finds an earthworm to eat, the
inner part of the mouth cavity is pushed out
and can engulf part of the worm. Large toothlike
structures inside the mouth catch and tear
at the worm. The specimen in the photo was
doing this when I caught it. Unfortunately to
photograph it, I had to place it in a container
to take it home and it surrendered the
earthworm. I then went in search of another
earthworm. Meanwhile the beast escaped
from my container, and I was left with only a
photograph in its memory.
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Downtime 
