76 ZOOLOGY. 
of plants consists of inorganic substances, decomposition necessarily taking 
place before any organic substance can afford food for a plant. 
_ Other peculiar characteristics of animals readily present themselves to 
the mind, but are not universal like these. Thus, animals are generally 
endowed with the power of locomotion, but some are destitute of it 
during the greater part of their existence. The animals with which we 
are most familiar all possess the senses of sight, hearing, &c.; but there 
are multitudes of creatures low in the scale of animal life which are. 
destitute of these, having no brain nor manifest nervous system, and no 
organs of special sense, although it seems that they are generally sensible 
of the contact of external objects, and they manifest an avidity for food. 
Animals generally have a mouth and a stomach—indeed, some of the 
lower kinds may be described as consisting of little more, and very much 
resembling a bag, the mouth of which is capable of being opened and shut 
; —but there are some which have 
neither mouth nor stomach, pro- 
perly so called, and imbibe 
their nourishment through their 
skin at all parts of their body. 
The animalcules of the genus 
Ameba' or Proteus? resemble 
small masses of jelly, which 
seem to flow rather than crawl 
or glide over objects. For loco- 
Fig. 62.—Amceba radiosa. motion they protrude a portion 
of the substance of the body in 
any direction, the remainder 
following it. When one of them finds a particle of matter suitable for 
food, it envelops it, the body opening at any point to receive it, closing 
round it again, and retaining it till all that is capable of assimilation is 
extracted from it, and absorbed into the living jelly. 
In all probability the largest animals that exist are certain species of 
whale ; but we are acquainted with many animals so small, that their 
existence can only be discovered by the aid of a powerful microscope, 
myriads inhabiting a single drop of water, whilst there may be. others 
still more minute, which no optical instrument yet invented enables us 
to discern. ‘Take any drop of water from the stagnant pools around us,’ 
says Professor Rymer Jones, ‘from our rivers, from our lakes, or from the 
vast ocean itself, and place it under the microscope ; you will find there- 
in countless living beings moving in all directions with considerable 

a, young Amoeba; 0, another specimen. 
1 Greek, ‘ vicissitude.’ 
2 From Proteus, a being of the Homeric mythology, who sought to elude observation by 
continually changing his shape, 
