20 NATURAL HISTORY. 
16. The third division of the animal kingdom is that 
of the Mollusks. This term comes from the Latin word 
mollis, soft. Mollusks are soft animals, most of them 
being inclosed in a hard shell, as the oyster, and all the 
varieties of shell-fish; and others bemg naked, as the 
slug. The central organs of the nervous system are 
ganglia variously arranged in the different orders of 
these animals. 
17. The fourth sub-kingdom of animals is that of the 
Radiates. In Fig. 9 you have a representation of one 
* if 
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Fig. 9.—Star-fish. 
of these animals, the star-fish, which will show you why 
they are called Radiates. You see parts extending like 
rays from the central portion. Radius is the Latin 
word for ray, and hence the name Radiate. 
18. It is the upper side of the star-fish that you see in 
this figure. On the under side it has a mouth in the 
centre. The arrangement of its nervous system is sin- 
