. Comparative anatomy and physiology. 278 COMPARATIVE ANATOMY AND Even naked Protozoa may become covered with a firm cyst formed by the ectosarc, at such times, as from choice or necessity they pass into a quiescent con- dition. This power of on cyst at ion is found also in the lowest members of the vegetable kingdom, and is a means of protection for the protoplasm at the time that it is undergoing the important changes that pre- cede the rejuvenescence of the indi- vidual, or the production of progeny. It is impossible to pass from the Protozoa without reminding the stu- dent of
. Comparative anatomy and physiology. 278 COMPARATIVE ANATOMY AND Even naked Protozoa may become covered with a firm cyst formed by the ectosarc, at such times, as from choice or necessity they pass into a quiescent con- dition. This power of on cyst at ion is found also in the lowest members of the vegetable kingdom, and is a means of protection for the protoplasm at the time that it is undergoing the important changes that pre- cede the rejuvenescence of the indi- vidual, or the production of progeny. It is impossible to pass from the Protozoa without reminding the stu- dent of how large and important a part they have played and .are playing in the formation of the earth's crust. The aphorism of Linnseus, " Petrefacta montium calcariorum non filii sed parentes sunt, cum omnis calx oriatur ab animalibus," is supported by our recently acquired knowledge that Diatoms and Globigerinse live on the surface of the sea, and that their cases and tests sink to the bottom when their inhabitants and makers die. Some rocks, such as chalk-cliffs, are full of the tests of Globigerinse, and the " Numuiulitic Limestone" of Nummulites. Casts of Foraminifers have been found in ^reensand ; a silicate of iron and alumina has been found filling casts of recent Foraminifera, so that as ct matter of fact we at this present period find " greensand replacing and representing the primitively calcareo-siliceous ooze;" and, lastly, the researches of the Challenger show that at a depth greater than 2,500 fathoms a substance known as " red-clay " takes the place of the Globigerina ooze. In all but the lowest Sponges (Myxospongise) skeletal structures have been observed, and these, as in the Protozoa, are of an organic nature simply (fibrous Fig. 113.—Tintin- nus lagenula, showing the Lorica below, and the Crown of Please note that these images are extracted from scanned page images that may have been digitally enhanced for readabili
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