348 ELEMENTARY BIOLOGY. [CHAP. 



a. Form. 



a. The body proper ; cylindrical, varying much in length 

 and diameter with the state of extension of the 

 animal j its conical free end (hypostome) with an 

 opening (mouth} at its summit. 



It is often difficult to observe the mouth, especially 

 in the green species. It may be readily seen however 

 if a Hydra, placed in a drop of water without a 

 coverslip, be watched under an inch objective, until 

 it turns its anterior end up towards the observer. 

 Occasionally, under these circumstances, the hypo- 

 stome may be everted. 



ft. The tentacles : ranged round the mouth ; their num- 

 ber and shape ; their varying length and diameter ; 

 the knob-like eminences on them. 



Look for developing tentacles, shorter and more 

 rigid than the rest. Very rarely, one or more may 

 branch. 



y. The base (so called foot) : a flattened disc ; narrower 

 or wider than the body, according to the state of 

 extension of the latter. It is functional as an organ 

 of adhesion. If the animal be attached, its se- 

 cretory product can often be seen, as a transparent 

 laminated mass, interposed between the body and 

 the surface of attachment ; if the animal be free, the 

 disintegrating remains of the same are often to be 

 found adherent to the 'foot'; if it be forcibly de- 

 tached, rupture of the base of the polype may ensue, 

 the animal appearing to possess that which has 

 been erroneously termed an 'anus'. 



& The buds; young Hydrae, of various sizes and stages 

 in development, attached to the sides of the parent. 

 There may be one or more of them. 



