14 OUTLINES OF NATURAL HISTORY. 



streams and ponds. Tliongli not very much larger than 

 the head of a large pin when contracted, the animal can 

 push itself out to a considerable length, and is generally 

 easily recognised by its brownish- red or orange colour. 

 The Hydra, lastly, has a great power of resisting mutila- 

 tion or mechanical injury. If cut up with a knife, all 

 the pieces will grow and develop themselves into fresh 

 Hydrce, and the animal may even be turned inside out, 

 without appearing to suffer thereby. 



Recapitulation op Essential Characters. — The 

 body is composed of two distinct membranes or layers, 

 an outer and an inner, of which the outer is furnished 

 with the offensive weapons known as thread-cells. There 

 is a mouth, surrounded by tentacles; but the mouth 

 opens into a large chamber, -which may be regarded as 

 the stomach and body- cavity in one. There are no 

 distinct organs of circulation, or respiration, and no 

 traces of a nervous system. These characters distin- 

 guish the Hydrozoa as a whole. 



CHAPTER V. 



class actinozoa. 



The chief animals comprised in this class are the so-called 

 Sea- Anemones and Corals ; and the scientific name of the 

 class is derived from the fact that the body generally 

 shows a distinctly star-like arrangement of its parts, 

 all of which "radiate" from a common centre (Greek, 

 aktin, a ray; zoon, an animal). They are therefore 

 "radiated" animals, and a good example of the class 

 may be found in the Actinia mesembryanthemum, one 

 of the commonest of the British Sea-anemones. 



This familiar Sea-anemone (fig. 5, A) has, when un- 

 disturbed and in a state of activity, the form of a short 



