INTRODUCTION. O 



Cienkowsky, Verworn, Rlmmbler, Bloclimann, Penard, 

 Maggi, Cattaneo, and Dangeard ; whilst Professor 

 Leidy, by the publication in 1879 of his classic ' Fresh- 

 water Rhizopods of North America,' stimulated observa- 

 tion in this department of zoology in his own country. 



PEOTOPLASM AND CELL- STRUCTURE. 



The constituent element in the composition of the 

 Rhizopoda is protoplasm, the living matter, as physio- 

 logy teaches, " from which all animated beings are 

 formed and developed, and to the properties of which 

 all their functions refer." They present no differentia- 

 tion of tissues or of organs ; their bodies are contractile, 

 and for the most part translucent, resembling, as has 

 been aptly said, a tenacious mucus, or soft tremulous 

 jelly ; whilst their movements, always slow and erratic, 

 seem aimless, except for the supply of the primary 

 need of an animated being the acquisition of food. 



The Rhizopoda, in common with all primitive life- 

 forms, are unicellular. It is desirable, in order to 

 make clear the significance of this term, to say a few 

 words regarding the structure, etc., of the simple cell. 



The cell, physiologically, is a minute vesicle, or closed 

 sac, the enveloping membrane or cell-wall enclosing 

 the protoplasmic substance in which the functional 

 phenomena reside. Of such cells modified, of course, 

 and more or less differentiated the bodies of all 

 animals are built up. A recent American writer* 

 thus summarises the ascertained facts in this connection. 

 The protoplasm of an ordinary typical cell in the 

 Metazoa, as well as in the higher plants, is differentiated 

 into cell-body or cytoplasm, and nucleus, of which the 

 difference in chemical composition is considerable. The 

 former is rich in proteids (albumen playing the most 

 important part) and poor in phosphorus. The nucleus, 

 on the other hand, is rich in phosphorus bound up in 

 a substance called nuclein, but poor in albumen. 



* Dr. G. N. Calkins, 'The Protozoa' (1901), chap. viii. 



