78 ZOOLOGY. 
Animals, contains animals which, however great their other differences, 
have this character in common, that they are composed of segments 
articulated or jointed together in a line. Insects ; Crustaceans, as crabs 
and lobsters ; Arachnids, as spiders and mites; and Annelids, as leeches 
and worms, belong to this division. These three great divisions continue 
to be recognised as Cuvier established them. Not so, however, his last 
great primary division, Radiata! or Radiated Animals, which takes its 
name from the arrangement of the organs of sense and motion in rays 
around a centre. Cuvier placed in this division many groups of animals 
in which no such arrangement is to be discerned, and which differ very 
widely from the true Radiata. With regard to these—the lowest tribes 
of animals—the progress of discovery has recently been very great, 
consequently a modification of Cuvier’s system has become necessary, and 
new groups have been constituted. 
Protozoa. 
The very lowest tribes of animals, those of most simple organisation, 
are now generally regarded as a distinct primary division of the animal 
kingdom, to which the name Protozoa? is given. With the exception of 
sponges, almost all the Protozoa are minute, and most of them are 
microscopic. Except a few that are found in the bodies of other animals, 
they all live in water. In general, their bodies consist simply of a mass 
of gelatinous matter. They have no nervous system, and therefore no 
organs of special sense; and none of them, except one group, have a 
mouth or an intestinal canal. 
Rhizopoda.—Among the lowest of the Protozoa, those most remarkable 
for the simplicity of their organisation, or want of distinct organs, are the 
Rhizopoda,’ a class particularly important on account of their universal 
diffusion, the multitudes in which they exist, and the great variety of 
kinds. The Amceba, already mentioned, may be regarded as a type of 
this class. All of them have, like it, the power of throwing out in any 
direction processes of the gelatinous substance of which they are composed, 
which are sometimes broad and lobe-like, but in other species are slender 
and of great length. The most interesting of the Rhizopoda are those 
which constitute the group called Foraminifera,* which are covered with 
a shell, and to which we owe the countless multitudes of minute fossil- 
shells forming great part of chalk and some other rocks, whilst the shells 
of still existing species abound in the sand of our sea-shores. These 
creatures are merely little particles of a kind of jelly; yet the shells 
which they produce are of the most exquisite beauty, and exhibit a 
1 From Latin radius, a ray. 2 From Greek protos, first, and z0on, an animal. 
3 From Greek rhizon, a root, and pous, podos, a foot. 
4 From Latin foramen, a pore, and fero, to bear. 

