CHAPTER II. 



THE HYDRA {HYDRA VIRIDIS, HYDRA GRISEA, ETC.). 



The Hydra is a small organism, not more than half an inch in 

 length when fully extended — and that would be a large specimen 

 — which is common in ponds and other pieces of still fresh 

 water. The animal exists in, at any rate, two forms in this 

 country ; one set of individuals are green, the others brown or 

 nearly colourless. Hydra has the general shape shown in the 

 accompanying figure (Fig. 6) ; it consists of a tubular body sur- 

 mounted by a wreath of tentacles which grow out from the base 

 of a conical expansion ; this bears the mouth at its summit. The 

 whole body is retractile, the retracted hydra having an oval to 

 round form. The hydra belongs to a large group of animals 

 containing the jelly-fish of our seas, the " Portuguese man-o'- 

 war," and a variety of soft-bodied creatures which, as will be 

 stated later, form one of the primary divisions of the animal 

 kingdom. This creature leads a stationary Hfe, adhering to a 

 leaf of duck-weed or a fragment of stone or stick, and by waving 

 its arms freely in the surrounding water, catches and narcotizes 

 by means of the thread cells — to be described presently — 

 minute worms, crustaceans, etc., which form its food. The 

 green hydra is coloured green by chlorophyll, a pigment which 

 is nearly universal in the vegetable kingdom, being only absent 

 in the fungi and in a few parasitic plants belonging to higher 

 groups. 



This chlorophyll is also present in a few other animals, even higher in 

 the scale than hydra, such as the two Planarian worms, Vortex vindis and 

 Convuluta schtihei. It also exists in a few Infusorians. It is not, however, 

 safe to jump to the conclusion that when an animal is coloured green it is 



