20 POPULAR ILLUSTRATIONS OF 



following order, beginning with what they consider the lowest form : 

 1. The Gregarina, 2. The Infusoria. 3. The Rhizopoda. 4. The 

 Eadiolaria. 5. Sponges. 



I shall have something to say about each of these classes when 

 their apparently unintelligible designations will become manifest ; 

 and first let us notice 



THE GREGARINA (from grego, to congregate). 



It will be better for me to say a few words here upon the dis- 

 tinction between animals and plants. And really this is a very 

 difficult question, and one by no means decided by naturalists ; for 

 although the most marked difference between the two organisms 

 is found in the evolution of oxygen and absorption of carbonic 

 acid by the plant, and the reverse by the animal, yet there are 

 exceptions even to this rule. Moreover, the rule itself does not 

 half help us in forming a differential diagnosis under the microscope. 



Some naturalists have founded the distinction upon the possession 

 of a mouth and stomach by animals, on the larger amount of nitro- 

 genous principle in their tissues, and also on their possession of 

 sensation and voluntary motion. But, as I shall soon show, there are 

 undoubted animals which have neither mouth, stomach, nor loco- 

 motive or sensitive organs. Again, the ascidian mollusk contains a 

 large amount of cellulose, a principle closely allied to starch. Starch 

 is also found largely in the solid textures of man and animals, while, 

 as we have seen, nitrogen exists in great abundance in the gluten 

 of cereals and other plants. 



The possession of sensation and voluntary motion is certainly 

 almost universal in animals, but it is by no means deficient in 

 plants, of which any one may satisfy himself by watching the move- 

 ments of Volvox globator and other low vegetable forms under the 

 microscope. In some of the Algse, also, there are distinct move- 

 ments observed in the granules which exist in the juices of the 

 plant. In the Confervae these granules are observed to move to- 

 wards a particular spot, where an aperture is formed, from which 

 they escape and move about in the surrounding water. Under a 

 high microscopic power these granules are seen to have 

 cilia or hair-like organs attached to them, by which 

 they move about in the water. They have the figure 

 as in margin. 



After a time the cilia disappear, and the granule fixes itself 

 to the side of the vessel, and becomes elongated, thus : 



It now forms a cell wall, and increases in growth by the 

 formation of other cells like itself. 



The great class of Diatomaceae a series of organisms whose 



Ife: 



