[Illustration:  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 (Ostracoda).  A few are larger forms, and belong to the same group as the existing brine-shrimps and fairy-shrimps (Phyllopoda).  One of the most characteristic of these is the Hymenocaris vermicauda of the Lingula Flags (fig. 32, d).  By far the larger number of the Cambrian Crustacea belong, however, to the remarkable and wholly extinct group of the Trilobites.  These extraordinary animals must have literally swarmed in the seas of