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 the later portion of this and the whole of
the succeeding period; and they survived in greatly diminished
numbers till the earlier portion of the Carboniferous period.
They died out, however, wholly before the close of the Palaeozoic
epoch, and we have no Crustaceans at the present day which can be
considered as their direct representatives.
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