278 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



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 eneystatioii 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 Linnaeus, " Petrefacta 

 montium calcariorum non filii sed 

 parentes sunt, cum omnis calx oriatur 

 Fig. us. Tintin- a b animalibus," is supported by our 



nus lagenula, . , -,-,-, -, 



showing the recently acquired knowledge that 

 and the Or ovm Diatoms and Globigerinae live on th, 

 of Cilia. surface of the sea, and that their case.s 



and tests sink to the bottom when 

 their inhabitants and makers die. Some rocks, 

 such as chalk-cliffs, are full of the tests of Grlobigerinse, 

 and the " Nummulitic Limestone " of Nummulites. 

 Casts of Foraminifers have been found in greensand ; 

 a silicate of iron and alumina has been found filling 

 casts of recent Foramlnifera, so that as a 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 (Myxospongiae) 

 skeletal structures have been observed, and these, as in 

 the Protozoa, are of an organic nature simply (fibrous 



