HYDRA AND CCELENTERATES IN GENERAL 117 



A number of species of Hydras are recognized by zoologists; 

 some of the characteristics of the principal ones are given in 

 Table IV. Hydra fusca has been selected as a type because it is 

 the species most easily obtained for laboratory use. 



External Characters. The body of Hydra fusca resembles an 

 clastic tube which varies in length and thickness according as 

 the animal is extended or contracted; in the former case it may 

 reach a length of 2 cm. At the distal end is a circlet of from six to 

 ten slender, finger-like projections called tentacles. The diameter of 

 the body is frequently increased at certain points by a distention 

 due to the ingestion of large particles of food. The general color 

 is brown. Usually two regions may be noted; a thin, nearly 

 colorless portion just below the tentacles and a thicker and more 

 deeply colored part extending to the opposite pole. The part of 

 the body which is usually attached to some object is known as 

 the foot or Ixisdl disk and is referred to as the proximal end. The 

 foot not only anchors the animal when at rest, but also serves as a 

 locomotor organ. A conical elevation, the hypostome, occupies 

 the distal end of the body. It is surrounded by the tentacles, 

 and has at the top an opening, the mouth. This mouth is not 

 the simple circular orifice often described, but is star-shaped, 

 having clefts running out from the center toward each arm 

 (104). 



The tentacles are capable of remarkable expansion, and may 

 stretch out from small blunt projections to very thin threads 

 7 cm. or more in length; in this condition they are so thin as to 

 be barely visible even with a lens. They move independently, 

 capturing food and bringing it into the mouth. Their number 

 varies as shown in Table IV. Reese (116) examined six hundred 

 specimens of Hydra riridis and found from four to twelve ten- 

 tacles on each. These occurred in the following proportions: 

 54 per cent had eight; 24 per cent, seven; 15 per cent, nine; 

 very u\\ animals possessed a greater number than nine, and only 

 occasionally was one found with less than seven. The number of 

 tentacles increase's with the size and age of the animal, although 



