252 COMPARATIVE ANATOMY 



surface the basal disc is detached and drawn up, and again fixed 

 close to the point where the mouth is attached. The mouth 

 then lets go, the body is again extended, a fresh hold is taken 

 by the mouth ; and so on. 



A living Hydra may easily be examined in a watch-glass full 

 of water under a simple lens or a low power of the miscroscope. 

 After being detached it soon fixes itself again to the sides of 

 the glass, and, if undisturbed, extends itself to a considerable 

 length, spreading out its tentacles like a net. 



Both body and tentacles are exceedingly contractile and 

 extensible. The body of the large species of Hydra fusca, when 

 fully extended, may attain a length of 7 or 8 mm. or more, 

 whilst its diameter is reduced in proportion as its length is 

 increased. The tentacles, when fully extended, project some 

 6 or 7 mm. beyond the body, and are reduced to the 

 thickness of the finest thread. When the animal is irritated 

 or alarmed, the body is contracted into a mere lump some 2 

 mm. long, and its diameter increases in proportion, whilst the 

 tentacles are reduced to circlets of blunt finger-like projections 

 surrounding the hypostome. All sorts of intermediate con- 

 ditions between the extremes of contraction and extension 

 may be observed. When the body is fairly extended it can 

 be seen that the trunk is not of uniform thickness but rather 

 club-shaped. The proximal moiety is slender and nearly 

 colourless, corresponding to the handle of a club, whilst 

 the distal moiety is thicker and more deeply coloured. The 

 external difference in form corresponds with an internal differ- 

 entiation of cell-structure, as will be described presently. 



Hydra is carnivorous, and feeds on the "water-fleas'' i.e. 

 small Crustacea, chiefly Daphnids and Copepods, which abound 

 in fresh-water pools. If some of these organisms are placed in 

 the watch-glass along with a Hydra, the manner in which the 

 animal captures and swallows its prey may easily be observed. 

 A Daphnid swimming within reach of the circle of extended 

 tentacles is seized and held fast by them. After a short 

 struggle it appears to be paralysed, and is drawn down- 

 ward towards the mouth by the contraction of the tentacles. 

 The hypostome being expanded and the mouth widely 

 opened, the Daphnid is slowly swallowed and passed down 

 into the digestive cavity. The distal moiety of the body 

 is often swelled up by the presence of one or more water- 



