CLASS POLYZOA. 47 



idle into three distinct regions — a head, thorax, and 

 abdomen. The head carries a single pair of feelers 

 (antennse), the organs of the mouth, and the eyes. _ The 

 thorax consists of three rings, and carries three pairs of 

 legs. Generally, the last two segments of the thorax carry 

 two pairs of wings. The adult insect breathes air directly, 

 and the respiratory organs are in the form of branching 

 breathing-tubes. The nervous system consists of a chain 

 of little nervous masses placed along the lower surface of 

 the body. The abdomen consists of distinct segments, 

 which do not carry locomotive appendages. These char- 

 acters distinguish the class of the Insecta as a whole. 



CHAPTER XIII. 



CLASS POLYZOA. 



This class includes the curious Sea-mosses and Sea-mats, 

 known technically as Polyzoa (Greek, jjoIus^ many ; zo'on, 

 animal), because they consist of colonies or assemblages 

 of little animals, associated so as to form compound 

 growths — much in the way that a tree is composed of 

 leaves and flowers supported upon a common trank. 

 As the type of this class we may take the broad-leaved 

 Sea-mat or Homwrack {Fhistra foliacea), which is of 

 common occurrence on the coasts of Britain. 



This singular organism (fig. 19, «) is extraordinarily 

 plant-like in form, and is generally regarded, when picked 

 up on the shore, as being a pale-brown sea-weed. It 

 forms a broad, and thin, leafy expansion, which is strongly 

 rooted below by a common stem to a stone or some other 

 foreign body, and which breaks up above into a number 

 of flattened branches. Its consistence is horny, and _ its 

 surface rough ; and when it is examined with a magnify- 

 ing glass, it is at once seen to be composed of an en- 



