CLASSIFICATION OF THE VERTEBRATA. 37 



and 120,000,000,000 of them, put together, would have no 

 more than a cubic inch in bulk ! And in the blood of the 

 Musk Deer, the red corpuscles are only one-fourth as large 

 as in Man. 



In the blood of Man the colorless corpuscles are larger 

 than the red corpuscles, and they are also more irregular in 

 form than the latter ; and what is especially noteworthy, is 

 the fact that they constantly change their form by what 

 may be called contraction and dilatation. Therefore it is 

 not strange that, as they float in the plasma of the blood, 

 and constantly exhibit themselves under new forms, they 

 remind the observer of the Amoeba and other similar low 

 forms of animal life. 



By diluting freshly-drawn blood with water, the colorless corpuscles 

 are killed, and distended by the absorption of water, perhaps, and then 

 their real structure is plainly seen. And they are found to be sacs having 

 thin walls, and containing a clear granular fluid, and a central body 

 or nucleus ; in a word, they are simply nucleated cells, differing in 

 no essential particular from the cells described on pages 12 and 13. 



As regards size, the representatives of the Branch of 

 Vertebrates, taken as a whole, are by far the largest of all 

 the members of the Animal Kingdom. There are many 

 kinds, however, that are very small. 



Although it is highly probable, if not really certain, that 

 the Vertebrates may be divided naturally into more classes 

 than are usually recognized, the most generally adopted 

 classification of this Branch of the Animal Kingdom recog- 

 nizes but four or at most five classes.* 



* Agassiz, in his " Essay on Classification," recognizes eight classes in the 

 Branch of the Vertebrates, thus : 



1. Mammals. 2. Birds. ?,. Reptiles. 4. Amphibians. 5. Solacliians 

 (Sharks, Skates, etc.). 6. Ganoids (Gar-pike, Sturgeon, etc.). 7. Fishes 

 proper. 8. Myzontes (Lampreys, etc.). 



It will be seen that this increase of classes results from dividing the old 

 group of Fishes into four groups. 



