IMPERFECTION OF PAL^EONTOLOGICAL RECORD. 57 



is sufficiently important to render it advisable to consider 

 each of the primary groups of the animal kingdom separately 

 from this point of view : 



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

 the entire classes of the Gregarinidce and Infusorian Animal- 

 cules, from their absence of hard parts, must ever remain un- 

 represented in a fossil condition. One or two of the latter, 

 however, possess an integumentary covering capable under 

 favourable circumstances of being preserved in rocks of 

 recent age. The Monera present no structures capable of 

 fossilisation ; and the same may be said of the Amcebea, 

 though one or two of the latter have a carapace which 

 might possibly be preserved. The remaining Rhizopodous 

 orders viz., the Foraminifera, Radiolaria, and Spongida 

 almost invariably develop hard structures of lime or flint ; 

 and all these orders, therefore, have left abundant traces of 

 their existence in past time. 



I. Coelenterata. Amongst the Ccelenterate animals, the 

 Fresh-water Polypes (Hydra) y the Oceanic Hydrozoa, the 

 Jelly-fishes (Medusidce), the Sea-blubbers (Lucernarida), the 

 Sea-anemones (Actinidce), and the CtenopJwra are destitute 

 of hard parts which could be preserved as fossils. The Sea- 

 blubbers, however, supply us with an instance of how a 

 completely soft-bodied creature may leave traces of its past 

 existence ; for there is no doubt that impressions left by the 

 stranded carcasses of these animals have been detected in 

 certain fine-grained rocks (the Lithographic Slate of Solen- 

 hofen). On the other hand, the coralligenous Zoophytes or 

 " corals " (comprising the Zoantharia sclerodermata and sclero- 

 basica, and most of the Alcyonaria) possess hard parts capable 

 of preservation, and the same is the case with most of the 

 Hydroid Zoophytes. Accordingly, there are few more abun- 

 dant fossils than corals ; whilst the large extinct group of 

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

 firs (Sertularians). 



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

 Echinodermata may be said to be represented more or less 

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

 oidea), however, the calcareous structures so characteristic of 



