660 GEOLOGY, 



As dynamic organisms animals have need lot supporting- and work- 

 ing-frames, for protective covering or housing, and for offensive and de- 

 fensive weapons, and these have been constructed chiefly out of inorganic 

 matter, and subordinately of indurated organic matter. It is through 

 these that animals have made their chief contribution to the material 

 of the geologic record. Skeletons and other hard parts to give internal 

 stiffness or firmness; shells, plates, indurated integuments, and various 

 other forms of external protection; teeth, spines, horns, and other means 

 of gathering and masticating food, and of attack and defense, contribute 

 material to the deposits, and form a record of the life activities and of 

 the physiographic environment. All of the eight groups of animals, viz. 

 Protozoa, Coelenterata, Echinodermata, Vermes, Molluscoidea, Mollusca, 

 Arthropoda, and Vertebrata, have left some record, but it is in all cases 

 a very imperfect one. 



The contribution of the Protozoa. — The Protozoa are related to the 

 animal kingdom much as the Thallophytes are to the vegetable, and 

 the two bear a close structural resemblance to one another. So near, 

 indeed, do the Protozoa and the Thallophytes approach one another in 

 their minuteness and simplicity, that the place of not a few organisms 

 is in doubt, and the two kingdoms, in general so different, seem here 

 to blend in the group Flagellata. The Protozoa are usually very minute 

 one-celled organisms with very little differentiation of tissue or organs. 

 Of the four classes of Protozoa, only one, the Rhizopoda, is found in the 

 fossil state. The rhizopods secrete silicious skeletons, and calcareous, 

 silicious, and chitinous tests of a great variety of forms, and this gives 

 them geologic importance. The deep-sea oozes and the chalk deposits 

 are their best-known contributions at present. They have probably 

 played a more important role in the formation of ordinary limestones 

 and silicious silts than can be demonstrated, because of the delicacy of 

 their relics and the ease with which these are pulverized by wave-action 

 in the shallow seas, or changed by recrystalhzation or by concretionary 

 aggregation. The globigerina oozes are formed largely from the cal- 

 careous shells of Foraminifera (Fig. 351), one of the orders of rhizopods, 

 among which the genus Globigerina is a leading form. Those forms 

 which make the deep-sea oozes live, not on the bottom, but near the 

 surface of the open sea, and on the death of the organisms, the shells, 

 tests, and skeletons sink to the bottom. Chalk is formed in a similar 

 way from calcareous Foraminifera, but not necessarily in very deep 



