12 



OUTLINES OF NATURAL HISTORY. 



The body of the Hydra (fig. 3, A) is in the form of 

 a simple cylindrical tnbe, the wall of which is composed 

 of two distinct layers, an outer and inner (shown by the 

 dark and light lines in fig. 3, B). At its base the body 



Fig. 3.— A, The Common Hydra (Hijdm vulr/aris), carrying young Hijdrce 

 which it has produced by budding, considerably magnified (after Hiuclfs); 

 B, Diagrammatic section of the Hydra, sliowing the mouth surrounded 

 by the tentacles, and the disc of attachment; the dark and light lines 

 indicate the two layers of the integument, and on one side of the body 

 is shown a single large egg. 



forms a little disc or sucker, by means of which the 

 animal can attach itself to stones, floating pieces of 

 wood, or the stems of water-plants. It can also, by 

 means of this disc, ghde slowly, like a snail, over the 

 surface to which it is fixed; but it can detach itself 

 entirely when it chooses. 



At the opposite end of the body is placed the opening 

 of the mouth, surrounded by from seven to twelve flexible 



