380 THE IMPERFECTION OP THE 



animals may be preserved as fossils. This subject is of sucli import- 

 ance that we may shortly consider each of the primary sub-divisions^ 

 of the ariimal kingdom separately from this point of vie-w. 



a. Protozoa: — As regards the sub- kingdom of the Protozoa, the 

 entire classes of the Infusorian Animalcules and the parasitic Grega- 

 rines, from their absence of hard parts, must ever be unrepresented 

 in a fossil condition. The same may be said of the Monera and 

 Amcebea, though one or two of the latter are provided with structures 

 which it is just possible might be preserved. The other three 

 Rhizopodous orders, viz., the Foraminifera, the Radiolaria, and the 

 Spongida are composed of organisms in which hard structures of lime 

 or flint are generally developed, and all these orders, therefore, have 

 left traces of their existence in past time. 



h. C odenterata : — Amongst the Goslenterate Animals, the Fresh- 

 water Polypes {Hydra), the Oceanic Hydrozoa, the Jelly-fishes 

 IMedusidce), the Sea-blubbers {Liicernarida), the Sea-anemones 

 {ActinidcB), and the Gtenophora are all destitute of hard parts which 

 could be preserved as fossils. The Sea-blubbers, however, supply us 

 with an instance of how even a completely soft-bodied creature may 

 leave traces of its former existence ; for there is no doubt that impres- 

 sions left by the stranded carcases of these animals have been detected 

 in certain fine-grained rocks (e. g. the Solenhofen slates of Bavaria).. 

 On the other hand the Ooralligenous Zoophytes, or " corals," [Zoan- 

 tharia sderodermata and sckrobasica and most of the Aloyonaria) 

 possess hard parts capable of preservation ; and the same is the case 

 with most of the Hydroid Zoophytes. Accordingly there are few 

 more abundant fossils than corals, whilst the large extinct group of 

 the Graptolites is generally placed in the neighborhood of the Sea- 

 firs (Sertularians). 



c. Annuloida .-—In this sub-kingdom the great class of the Eehino- 

 dermata may be said to be represented more or less completely by all 

 its orders. In the Sea-cucumbers {Holoikwroidea), however, the 

 calcareous structures so characteristic of the integument of the other 

 Echinoderms are reduced to their minimum or are wholly wanting ; 

 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., that of the Scoledda) comprises animals which are 

 without exception destitute of hard parts, and which in many cases 

 live parasitically in the interior of other animals (e. g., the Tape- 



