IMPERFECTION OF PAL^ONTOLOGICAL RECORD. 29 



completely by all its orders. In the Sea-cucumbers (Holothur- 

 oidea\ however, the calcareous structures so characteristic of 

 the integuments of the other Echinoderms are reduced to their 

 minimum ; and accordingly the evidence of the past existence 

 of these creatures is of the most scanty description. The other 

 great class of the Annuloida (viz., the Scolecida) comprises 

 animals almost without exception destitute of hard parts, and 

 which mostly live parasitically in the interior of other animals 

 (e.g., the Tape -worms, Suctorial -worms, Round -worms, &c.) 

 We are therefore without any geological evidence of the former 

 existence of Scolecida, though no doubt can be reasonably 

 entertained but that the group dates back to a time long an- 

 terior to the present fauna. 



d. Annnlosa. Many of the lower Annulose animals, such 

 as Leeches, Earth-worms, and Errant Annelides, possess no 

 structures by which we could expect to get direct evidence of 

 their past existence. The last of these, however, have left 

 ample traces of their former presence in the form of burrows 

 or " tracks" upon the mud and sand of ancient sea-bottoms ; 

 and the so-called " Tubicolar " Annelides are well represented 

 by their investing tubes. In the case of the higher Annulosa, 

 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, further, are not only aqueous, but 

 are also marine, most fossils are those of sea -animals. It 

 follows, therefore, that the remains of air-breathing animals, 

 whether these be terrestrial or aerial, can only be preserved in 

 an accidental manner, so to speak ; except the animal inhabit 

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

 which 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 Crustaceans ; since these not only have a resisting shell or 

 " exoskeleton," but are also generally aquatic in their habits. 

 The air-breathing classes of the Myriapoda (Centipedes and 

 Millipedes), the Arachnida (Spiders and Scorpions), and the 

 Insect a or true Insects, on the other hand, have been much 

 less commonly and completely preserved, though many of 

 them are perfectly capable of being fossilised. Almost all such 

 remains, however, 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 



