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tion, as they have no permanent existence. There are no 
digestive organs, no circulatory system, no nervous system, 
no reproductive organs. As in all living beings, the processes 
of growth and the maintenance of the body are effected by 
intussusception and assimilation : that is to say, foreign 
matter is conveyed from the exterior to the interior of the 
animal, and is then submitted to certain influences by which 
it is “ assimilated,” or made like to the matter composing the 
animal which it is about to nourish. Finally Protomyxci has 
the power of reproducing itself by a process of “ encystation,” 
quite similar to what is known to occur in various of the 
higher Protozoa. It is unnecessary to enter here into the 
details of this process, but it consists essentially in the assump- 
tion by the adult organism of a quiescent stage, in which its 
pseudopodia are all withdrawn, and the protoplasm becomes 
surrounded by a thick hyaline capsule or cyst. The contained 
protoplasm ultimately breaks up into a number of minute 
spherical balls, without any outer envelope, which are liberated 
by the rupture of the outer cyst, and after a short period of 
independent life are developed into as many new individuals. 
In such a Moner as Protomyxa we find the very simplest 
expression of the great equation of life, and, as before re- 
marked, no type of structure so simple and so undifferentiated 
is to be found elsewhere among animals, or, except temporarily, 
in the vegetable kingdom. All other animals and all adult 
plants consist of single or aggregate masses of protoplasm in 
the form of “cells,” properly so called. What is termed a “cell” 
is, as is well known, a mass of protoplasm, usually of a more or 
less definite shape, typically spheroidal, enclosing in its interior 
a distinct vesicular, or solid, variously-shaped body, which is 
shown by the readiness with which it is stained by carmine 
to be also of a protoplasmic nature, and which is termed the 
“ nucleus.” Such a nucleated mass of protoplasm constitutes 
a single “ cell,” and it may or may not be enclosed in a thinner 
or thicker external covering, which differs in consistence, and 
often in chemical composition, from the protoplasmic contents, 
which may be rigid or flexible, and which is known as the 
“ cell-wall.” 
A single independent cell may alone constitute an individual 
animal or plant ; or an animal or vegetable may be an aggre- 
gate of cells, variously disposed, and variously modified ; or, 
lastly, an animal may be an aggregate of cells, of which some 
form the actual tissues, and have definite places and relations 
with one another ; while others are locomotive, and, from one 
point of view, semi-independent. We may briefly consider 
cases of each of these conditions. As an example of the case 
