ECHINODEEMA — SEA-CUCUMBEES — ANNELIDA. 77 



no representatives among freshwater fossils. They are all G-allery 

 soft-bodied animals, and the only portions capable of "VIH. 

 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 were 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 " 

 which are now believed to have been formed by other 

 animals, such as arthropods or molluscs. Though it may 

 sometimes be convenient to give them names, it must be 

 remembered that this implies no knowledge of the animal to 

 which they may have been due. The most abundant fossils 

 assigned to these Annelida are the hard tubes which the 

 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 they 

 were always formed by animals related to the modern 

 makers of similar tubes — the Polychaeta Tubicola. More- 

 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- Cambrian age at Loch 

 Fyne in Argyll. Some large slabs presented by the eighth 

 Duke of Argyll are in Wall-case 8 of Gallery XI. ; smaller 

 specimens are with the other British Annelida in the present 



