84 AN INTRODUCTION TO ZOOLOGY. 



sive joints of their back-bones and successive pairs of 

 nerves branching from the central nervous axis. 



How this kind of symmetry first arose is quite un- 

 certain ; but it is the predominating type of the animal 

 kingdom^ intimately correlated with the power of 

 moving from place to place ; while the radial symmetry 

 is a restricted and therefore comparatively unimport- 

 ant type, and the bilateral type, without any trace of 

 metamerism, is almost confined to the lower groups of 

 worms. 



Something has already been said about the limbs 

 of animals, which, in the case of typical segmented 

 animals, are called appendages. The majority ot 

 animals have limbs of some kind. Unicellular 

 animals, such as amoeba, put out temporary branches 

 of their protoplasmic substance, to catch or feel with, 

 and hence cells which can do so are called amoebiform. 

 These branches are called pseudopodia, or false feet. 

 Others of the Protozoa have long feelers of protoplasm, 

 which are fixed in shape. The Goelenterata have ten- 

 tacles, or feelers, fleshy arms surrounding the mouth 

 on all sides, which give them their radiate appearance, 

 and are often endowed with special stinging cells, which 

 paralyse the prey they catch. The higher segmented 

 animals present an amazing variety of limbs, as in 

 the case of the lobster already mentioned, or the 

 typical insect. The latter has three pairs of walking 

 legs, and aniennce or feelers on the head, besides 

 movable jaws. 



In speaking of the limbs or appendages of an animal. 



