RELATION' OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 259 



careous and much like that of the Gasteropoda, and such shells as these 

 in considerable variety are found among fossil faunas. Many aquatic 

 annelids burrow in the mud or sand at the bottom of the water in which 

 they live, and similar burrows arc not unfrequently found in sediment- 

 ary strata, which were doubtless made by extinct annelid species. 



Gcelenterata. — The Ccelenterata consist of somewhat numerous orders 

 and families, all of which are aquatic animals and all except a few in- 

 conspicuous forms are denizens of saline waters. Therefore if all the 

 living - members of this branch of the animal kingdom were possessed 

 of such hard parts as would resist decomposition after death, we would 

 be justified not only in inferring that the bodies of their extinct repre- 

 sentatives were similar in structure, but in assuming that all of them 

 have been more or less completely represented by fossil remains. .V 

 large part of the living Ccelenterata, however, are entirely destitute of 

 even the most delicate hard parts, while others secrete a more or less 

 massive calcareous or corneous skeleton, or sometimes an external 

 tube, such as the well-known corals, sea fans, etc. It is therefore 

 necessary to infer that while a very large proportion of the Ccelen- 

 terata which have existed during past geological time secreted coral- 

 line skeletons and tubes in infinite variety, another large proportion 

 have left no material proof of their existence. It is true that casts 

 and impressions in fine sedimentary strata of certain extinct forms of 

 jelly fishes have been discovered;* but this is a rare and remarkable 

 exception to the rule just referred to, the purport of which is that none 

 of the extinct animals whose bodies consisted only of soft parts could 

 have left any satisfactory evidence of their existence. Fossil corals 

 are often so well preserved that they may be as completely studied as 

 the now living forms, but still much of the structure of the extinct 

 polyps must forever remain unknown. 



Protozoa. — All the Protozoa to which reference need be made in this 

 connection are of aquatic habitat, most of them living in marine waters. 

 These are the Foraminifera, Radiolaria, and Spongida, and only these 

 secrete such hard parts as resist decomposition. Their hard parts 

 are sometimes in the form of minute complex calcareous or siliceous 

 shells, sometimes of calcareous or siliceous masses, and they sometimes 

 consist of the well-known substance which constitutes sponges. 



Much of protozoan life has no known connection with such hard parts 

 ashavejust been mentioned, and it is presumable that the proportion of 

 protozoan forms which secrete no hard parts was similar in past geolog- 

 ical epochs to that which now obtains. If so, there must have been an 

 abundant representation of such life in the past of which no trace has 

 been left. 



Plants. — The natural method by which plants or portions of them 

 maybe preserved from decay, and the conditions that during the geo- 



* Sec remarks on eon<l.it ions of* lossili/ation at the end of this ess;iy, 



