64 INTRODUCTION. 



an integumentary covering which under favourable circumstances is 

 capable of preservation, and remains of these are believed to occur 

 in some of the later sediments. Of the Rhizopods, the Monera and 

 the most of the Amoebea have no hard structures, and are unknown 

 as fossils, but a few of the latter possess a " test " which might pos- 

 sibly be preserved in the fossil condition. On the other hand, 

 skeletal structures of lime or flint are almost always developed in the 

 great Rhizopodous orders of the Foraminifera and Radiolaria ; and 

 these groups, therefore, have left abundant traces of their existence 

 in past time. 



b. Port/era. — A few of the Sponges (Myxospongice) are completely 

 destitute of hard parts, and have therefore no palaeontological 

 history. The greater number of the Porifera, however, possess a 

 calcareous or siliceous skeleton, which is more or less capable of 

 fossilisation, and the record of such types is therefore a long and 

 comparatively full one. 



c. Calenterata. — Amongst the Ccelenterate animals, the Fresh-water 

 Polypes (Hydra), the Oceanic Hydrozoa, the Jelly-fishes (Medusidce), 

 the Sea-blubbers (Lucernarida), the Sea-anemones (Actinidce), and 

 the Ctenophora are destitute of hard parts which could be preserved 

 as fossils. The Jelly-fishes and Sea-blubbers, however, supply us 

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

 traces of its past existence ; for impressions left by the stranded car- 

 casses of these animals have been detected in certain fine-grained 

 rocks (e.g., the Lithographic Slates of Solenhofen). On the other 

 hand, the coralligenous Zoophytes or " Corals " (comprising the 

 Madreporaria and most of the Alcyonarid) possess hard parts cap- 

 able of preservation, and there are therefore few classes of organisms 

 which are more fully represented in past time than the true Corals. 

 As regards the Hydrozoa, most of the Hydroid Zoophytes have a 

 horny skeleton, and do not become readily fossilised, though the 

 large extinct group of the Graptolites is generally regarded as allied 

 to the recent Sertularians. In a few of the Hydracttntdce, and in 

 the whole group of the Hydrocorallines, a calcareous skeleton is 

 present, and various fossil forms of these are known. In the great 

 extinct group of the Stromatoporoids, now usually referred to the 

 Hydrozoa, the skeleton was also of a calcareous nature. 



d. Echinodermata. — Since the integument of the Echinoderms is 

 liable to undergo more or less extensive calcification, this great divi- 

 sion of animals is represented more or less completely in past time 

 by all its sections. In the group, however, of the Sea-cucumbers 

 (Holothuroidea), the calcareous structures so characteristic of other 

 Echinoderms are reduced to their minimum ; and accordingly, the 

 evidence of the past existence of these animals is of a very limited 

 description. 



