CLASSIFICATION OF ANIMALS. 21 
gular. It is seen in Fig. 10. The place of the mouth is 
indicated at a@ Around this is a 
nervous cord connecting together 
five ganglia, which are at the be- 
ginnings of the five arms of the an- 
‘imal. From each ganglion a nerve 
goes along each arm ending at its 
point in what is supposed by some 
‘ i to be a kind of eye. Though the 
Fig. 10.—Nervous System of @nimals of this sub-kingdom have 
Star-fish. great variety of form, the arrange- 
ment is essentially the same as that which you see in this 
animal. 
19. These four sub-kingdoms are arranged in the order 
of their rank; the highest, or rather the most compli- 
cated, being placed first, and the simplest last. This is 
true of them in the general, and yet there are some in any 
one of the three lower divisions or groups that are high- 
er in organization than some of the simplest in the one 
just above it. In the lowest group, the radiate, there 
are some animals which are nothing but a stomach with 
an apparatus to put food into it. The animals of one 
group are sometimes said to be more perfect than those 
of another; but this is not true, for the organization of 
every animal is perfectly adapted to its wants and its 
mode of existence. 
20. There are many terms used in classifying the ani- 
mals of each sub-kingdom, which you should understand 
at the outset. All animals that come from a common 
origin or parentage are said to belong to the same species. 
Thus all men descended from Adam, and therefore be- 
long to one species, although they differ from each other 
in different quarters of the earth. These differences 
arise from accidental causes, as climate, food, habits, ete., 
and are not therefore specific differences. They make 
mere varieties, and not different species. So dogs and 
horses belong to two different species; but there are 
