Natural Science Forum / Biology / Paleontology / October 2005

The bulbous thing isn't a head. Or a balloon. It's not even bulbous, and
in fact it's not even a thing. What you are seeing is some artist's
impression of what is actually an amorphous cloud of organic stuff
emitted from the anus, either at death or afterwards. The head is at the
other end, and the flaccid tentacle snouts are legs. There are not just
7 of them, there are 7 pairs. The beast is a spined velvet worm
(onychophoran).

There's a long story on Hallucigenia and it may not be finished.

When Simon Conway-Morris set out to redescribe the critter a couple of
decades ago, he had about 30 Burgess Shale specimens to work with.
There was no indication of which end was the front, and which side was
the top.  He did the best he could with the improbable critter,
deciding more or less arbitrarily that the blob that appears in a
number of specimens was the head even though it lacks features that
one would like to see on a head ... like a mouth for starters.  He
also decided that walking on the spines was (slightly) less improbable
than walking on the single row of tubes.  He even worked out a
plausible method of locomotion on the spines -- which, unlike the
tubes, are obviously paired.

In the 1990s, Ramiskold and Hou redescribed the animal based on
additional specimens from the Maotianshan Shale in Yunnan, China.
They dispensed with the "head" believing that it was, as John Harshman
says, a cloud of organic matter squeezed out ot the animal when it was
buried.  This turned the critter around, making the head the tail.
That's fairly non-controversial.  The legs are somewhat more of a
probleml.  None of the dozens of known specimens of Hallucigenia
actually show paired tubes (legs).  The evidence for pairing comes
from dissecting through a Chinese specimen and finding what is thought
to be a second leg under it.  No one has really explained how it is
that none of the dozens of known specimens show paired legs even
though some do seem to show pairing of the tendrils in front of the
legs (or behind them in the old model).  Ramiskold and Hou remarked on
that in their paper redescribing the animal.

Ramiskold an Hou had one additional thing going for their
interpretation which is that the Maotianshan Shale fauna includes
several "armoured worms" that aren't present in the Burgess fauna.
Some of these -- notably Microdictyon -- could, if you work at it, be
hypothesized as distant relatives of Hallucigenia.  If they really are
related, then the tubes of Hallucigenia probably are feet.

For more details, see
http://www.wikipedia.org/search-redirect.php?search=hallucigenia&language=en&go=Go