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 

 to be a kind of eye. Though the 

 Fig. lo.-NenJous'system of animals 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 Li 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, etc., 

 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 



