58 INTRODUCTION. 



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 evi- 

 dence of the former existence of Scolecida, though no doubt 

 can be reasonably entertained but that the group dates back 

 to a time long anterior to the present fauna. 



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

 as Leeches, Earth-worms, and Errant Annelides, possess few 

 or 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 are known also by their horny jaws ; while 

 the so - called " Tubicolar " Annelides are well represented 

 by their investing tubes. In the case of the higher An- 

 mdosa, 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 Insecta or true 

 Insects, on the other hand, have been much less commonly 



