IMPERFECTION OF PAL^ONTOLOGICAL RECORD. 65 



e. Annulosa. — The lowest division of the Annulosa (viz., that of the 

 Scolecida) comprises animals almost without exception destitute of 

 hard parts, and mostly living parasitically in the interior of other 

 animals. With the exception, therefore, of some excessively prob- 

 lematical fossils which have been supposed to indicate the past exist- 

 ence of Ribbon-worms (Nemer leans), the palseontological history of 

 the Scolecida is a total blank. Most of the true Worms {Anarthro- 

 podd) possess few or no structures by which we could expect to get 

 direct evidence of their past existence. The Polychaetous Annelides 

 have, however, left ample traces of their former presence either by 

 their horny jaws or by means of their investing tubes. There are 

 also numerous fossils of the nature of burrows or " tracks " upon the 

 mud or sand of ancient sea-bottoms, which have been regarded, with 

 more or less probability, as having been produced by Annelides. 



In the case of the higher Anmdosa (Artlzropoda), another law 

 steps in to regulate their comparative abundance as fossils. Most, 

 in fact almost all, fossiliferous formations have been deposited in 

 water ; and of necessity, therefore, most fossils are the remains of 

 animals whose habits are naturally aquatic. As most deposits, fur- 

 ther, are not only aqueous, but are also marine, most fossils are those 

 of sea-animals. It follows, therefore, that the remains of air-breath- 

 ing animals, whether these be terrestrial or aerial, can only be pre- 

 served in an accidental manner, so to speak — except the animal 

 inhabit water (as the Cetaceans do), or except in the rare instances 

 in w T hich old land-surfaces have been buried up by sediment, and 

 thus partially kept for our inspection. In accordance with this law, 

 the most important and abundant fossil Annulose animals are Crus- 

 taceans; since these not only have a resisting shell or " exoskeleton," 

 but are also generally aquatic in their habits. The air-breathing 

 classes of the Myriopoda (Centipedes and Millepedes), the Arach- 

 nida (Spiders and Scorpions), and the Insecta or true Insects, on the 

 other hand, have been much less commonly and completely pre- 

 served, though many of them are perfectly capable of being fossil- 

 ised. Almost all such remains, moreover, as we have of these three 

 great classes, are the remains of isolated individuals, which may have 

 been accidentally drowned ; or else they occur in hollow trees, or in 

 fragments of ancient soils, or in vegetable accumulations such as 

 coal and peat. There is, however, a considerable number of aquatic 

 insects (but almost exclusively in fresh water), and there are many 

 insects the larvae of which inhabit water, whether this be fresh or salt ; 

 so that instances of these occurring as fossils are not very infrequent. 



f. Molluscoidea. — With regard to the Molluscoids, the great majority 

 of the Polyzoa and all of the Brachiopoda possess skeletal structures 

 of a horny or calcareous nature, which are capable of preservation in 

 the fossil condition. Both these classes of animals, therefore, possess 



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