INTRODUCTION. 3 
Cienkowsky, Verworn, Rhumbler, Blochmann, 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. 
PROTOPLASM 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. 
