17 



CHAPTEK II. 



PROTOZOA, OR SINGLE-CELLED ANIMALS. 



The Protozoa are the simplest forms of animal life : they are all 

 of small size, of .extremely simple constitution, and invariably 

 unicellular. In no case do we find sexual reproduction or 

 sexual differences of any kind. They are animals that have 

 remained as simple cells, to aU intents and purposes like the cell 

 described in chap. i. Some forms of protozoa are simple drops 

 of sarcode — protoplasm ; others have not only a definite cell- 

 wall, but possess the power of secreting calcareous and siliceous 

 shells. These shell-bearing species, or Foraminifera (tig. 3, iv. 

 and v.), are present in myriads in the waters of the ocean, their 

 " tests " or shells falling to the floor of the sea as the animals 

 die. Many of these tests are dissolved before they reach the 

 bottom, if the depth of water be very great ; yet millions of 

 others arrive safely upon the bed of the sea, and there by slow 

 degrees they form a layer of a white or creamy colour. Of such 

 formation is the globigerine ooze on the floor of the Atlantic 

 and also the radiolarian ooze, — protozoa of the genera Globigerina 

 (fig. 3, V.) and Eadiolaria taking the chief part in the formation 

 of these two oozes respectively. Of ancient rocks we know 

 that much of the Chalk has been formed in a similar way, by 

 the slow accumulation on the sea-bed of these and other falling 

 tests. Not only do we flnd that the Chalk in many instances is 

 built up of these minute organisms, but also that their tiny shells 

 represent genera existing at the present day. What countless 



B 



