ECHINODEEMA — SEA-GUCUMBEES — ANNELIDA, 
77 
soft-bodied animals, and the only portions capable of 
preservation in the rocks are the bristles, used for locomotion, 
and the horny jaws. The bristles of course can only be 
identified when connected with other traces of the animal. 
The jaws, some of which in the fossil state Avere long known 
as conodonts, are so minute that they can as a rule only be 
found by the washing and microscopical examination of the 
softer rocks. Evidence for the former existence of the free- 
moving forms, known as Polychaeta Errantia, may also be 
furnished by impressions, borings, trails, or worm-castings, 
the last-mentioned being the mud passed through the 
animal’s body for the extraction of food and then excreted in 
coiled heaps (Fig. 37). These traces are somewhat unsatis- 
factory, and many have been vaguely assigned to “ worms ” 
Avhich are now believed to have been formed by other 
animals, such as arthropods or molluscs. Though it may 
sometimes lie convenient to give them names, it must be 
remembered that this implies no knowledge of the animal to 
which they may have been due. Tlie most abundant fossils 
assigned to these Annelida are the hard tubes which tlie 
sedentary forms build up, sometimes from sand-grains stuck 
together, sometimes of carbonate of lime deposited in layers 
by the skin. These, however, show so little characteristic 
structure or even shape that it is difficult to be sure that tliey 
were always formed liy animals related to the modern 
makers of similar tubes — the Polychaeta Tubicola. IMore- 
over, since .simple tubes are fashioned by some other kinds 
of animals, for example boring molluscs, one cannot even be 
certain that all these fossils are due to polychaetes. In 
spite of these difficulties, fossil “worms ” have some interest 
for the geologist, since many of them are sufficiently distinct 
to enable him to identify stratigraphical horizons by their 
means, while others have left their remains in such quantity 
as to build relatively large masses of rock, and others again 
throw light on the conditions under which the rock wherein 
they occur was deposited. 
The obscurest fossils of any that have been referred to 
Annelida are, as might be expected, also the oldest. They 
are, in fact, the oldest traces of life in the Museum, and come 
from rocks believed to be of Pre-Canabrian age at Loch 
Fyne in Argyll. Some large slabs presented by the eighth 
Duke of Argyll are in Wall-case 8 of Gallery XL; smaller 
specimens are with the other British Annelida in the present 
Gallery. With these latter are other obscure fossils, named 
Gallery 
VIII. 
