C(ELENTERATA 215 



ing a branching hydroid, one may often see small, cup- 

 like structures attached to the stem. These are the 

 hydrothecez, and in them are set the hydroid persons, 

 more or less resembling minute hydras. The reproduc- 

 tive persons may remain permanently attached to the 

 colony, or, in other species, they are set free as swim- 

 ming medusas, whereby the range of the species is ex- 

 tended. The word medusa, as applied to a jellyfish or The me 

 similar animal, is derived from the Medusa of ancient 

 fable, a woman with snakes for hair. The naturalists 

 of early times, who had a good deal of imagination, 

 fancied a resemblance between the head of the medusa 

 and the jellyfish, with its snakelike pendent tentacles. 

 When the reproductive person or medusoid is set free, 

 the base becomes the upper surface and the oral side is 

 below. These medusoids have been found and studied 

 by naturalists in many cases without reference to the 

 hydroid stage ; consequently two systems of classifica- 

 tion have sprung up for stages of the same animals. By 

 degrees, however, the connection between particular 

 colonial forms and their medusoicls is being established, 

 and the classifications are amended accordingly. 



Certain medusoids possess small vesicles at the mar- 

 gin of the bell or umbrella, and these vesicles contain 

 statoliths, - - hard, stony bodies which are supposed to 

 enable the animals to perceive their position in space. 

 The force of gravity, acting on the statoliths, produces a 

 downward pressure to which the animal reacts. Thus 

 the function of these organs is something like that of the 

 semicircular canals in the human ear, but in these latter 

 the mechanism is entirely different. Nature attains the 

 same or similar ends in wholly diverse ways. 



The Scyphozoa are not very closely allied to the Hy- 

 drozoa, and it is even probable that they acquired the fishes 



