STRUCTURE. 183 



shapes itself, are comparatively short and club-shaped; in 

 others they are long and fine filaments which anastomose, 

 so forming a network running here and there into little 

 pools of protoplasm. Then there are kinds in which the 

 protoplasm streams up and down the protruding spicules: 

 sometimes inside of them, sometimes outside. Always, too, 

 there is included in the protoplasm a small body known as 

 a centrosome. 



Lastly, we have the innermost element, considered the 

 essential element — the nucleus. According to Prof. Lan- 

 kester, it is absent from Archerina, and there are types in 

 which it is made visible only by the aid of special reagents. 

 Ordinarily it is marked off from the surrounding protoplasm 

 by a delicate membrane, just as the protoplasm itself is 

 marked off by the exoplasm from the environment. Most 

 commonly there is a single nucleus, but occasionally there 

 are many, and sometimes there is a chief one with minor 

 ones. Moreover, within the nucleus itself there have of 

 late years been discovered remarkable structural elements 

 which undergo complicated changes. 



These brief statements indicate only the most general 

 traits of an immense variety of structures — so immense a 

 variety that Prof. Lankester, in distinguishing the classes, 

 sub-classes, orders, and genera in the briefest way, occupies 

 37 quarto pages of small type. And to give a corresponding 

 account of Protophyta would require probably something 

 like equal space. Thus these living things, so minute that 

 unaided vision fails to disclose them, constitute a world ex- 

 hibiting \'arieties of structure which it requires the devotion 

 of a life to become fully acquainted with. 



§ 54&. If higher forms of life have arisen from lower forms 

 by evolution, the implication is that there must once have 

 existed, if there do not still exist, transitional forms; and 

 there follows the comment that there do still exist transi- 

 tional forms. Both in the plant-world and in the animal- 

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