Pay hl 63 NATURAL HISTORY. 
derful, can only be seen by the microscope; and this, 
from their transparency, is easily done. They are most- 
ly aquatic animals, and have one or two rows of cilia, or 
hair-like filaments. It is the motion of these that gives 
the apparent wheel-like rotation from whence their name 
<_ is derived. 
Questions.—What does the class Crustacea include? What gives 
them their name? In what respects are they like insects? In what 
element do most of them live? What peculiar provision is there in 
one of the exceptions to this? What is said of the legs of the Crus- 
tacea? What of their metamorphosis? What of their ring-like ar- 
rangement? What of the composition of their covering ? What is 
the necessity of its being shed from time to time? Describe the man- 
ner in which this is done in the Lobster. How many orders have the 
Crustacea? What animals are included in the order Decapoda? 
What are their peculiarities? What is said of the Shrimps and 
Prawns? What of the Hermit Crabs? What of the Whale Louse ? 
To what order belong the Barnacles and the Acorn-shells? Describe 
the construction and habits of the Barnacle. What are the charac- 
teristics of the class Annelida? How many orders has it? What is 
said of the order Dorsi-branchiata? What gives the name to the Tu- 
bicola? What is said of the Serpula? What of the Terebella ? 
What is said of the order Terricola? What of the eggs of the Earth- 
worm? What of the usefulness of those animals? What is said of 
the order Suctoria? What of the class Entozoa? What of the class 
Rotifera ? 
CHAPTER XXXII. 
MOLLUSKS. 
4542, Tur. animals of the sub-kingdom of the Mollusca 
or Mollusks are so named from the Latin word mollis, 
soft. Their bodies are soft, and moist, and cold, as you 
see exemplified in the Oyster and the Slug. All animals 
that live in shells, with some few exceptions, already no- 
ticed, belong to this sub-kingdom. But some belonging 
to it have no shelly covering, as the Slug and the Cuttle- 
fish, and these are said to be naked. 
