lustration: Fig. 30.--Annelide-burrows (_Scolithus linearus_)
from the Potsdam Sandstone of Canada, of the natural size. (After
Billings.)]

The Ringed-worms (_Annelida_), if rightly credited with all the
remains usually referred to them, appear to have swarmed in the
Cambrian seas. Being soft-bodied, we do not find the actual worms
themselves in the fossil condition, but we have, nevertheless,
abundant traces of their existence. In some cases we find vertical
burrows of greater or less depth, often expanded towards their
apertures, in which the worm must have actually lived (fig. 30),
as various species do at the present day. In these cases, the
tube must have been rendered more or less permanent by receiving
a coating of mucus, or perhaps a genuine membranous secretion,
from the body of the animal; and it may be found quite empty,
or occupied by a cast of sand or mud. Of this nature are the
burrows which have been described under the names of _Scolithus_
and _Scolecoderma_, and probably the _Histioderma_ of the Lower
Cambrian of Ireland. In other cases, as in _Arenicolites_ (fig.
32, b), the worm seems to have inhabited a double burrow, shaped
like the letter U, and having two openings placed close together
on the surface of the stratum. Thousands of these twin-burrows
occur in some of the strata of the Longmynd, and it is supposed
that the worm used one opening to the burrow as an aperture of
entrance, and the other as one of exit. In other cases, again,
we find simply the meandering trails caused by the worm dragging
its body over the surface of the mud. Markings of this kind are
commoner in the Silurian Rocks, and it is generally more or less
doubtful whether they may not have been caused by other marine
animals, such as shellfish, whilst some of them have certainly
nothing whatever to do with the worms. Lastly, the Cambrian beds
often show twining cylindrical bodies, commonly more or less
matted together, and not confined to the surfaces of the strata,
but passing through them. These have often been regarded as the
remains of sea-weeds, but it is more probable that they represent
casts of the underground burrows of worms of similar habits to
the common lob-worm (_Arenicola_) of the present day.

The _Articulate_ animals are numerously represented in the Cambrian
deposits, but exclusively by the class of _Crustaceans_. Some
of these are little double-shelled creatures, resembling our
living water-fleas (_Ostrac