l86 FRANK M. SURFACE. 



two balls are formed at the same time, thus dividing the young 

 colony. 



The ball of young individuals is spherical in shape and is very 

 regularly formed. Indeed, this is one of the very striking things 

 about these interesting balls. The feet of the individuals are not 

 attached together in an irregular mass, as one might suppose, but 

 they are arranged so as to form a very regular sphere. It is diffi- 

 cult to ascertain just how the animals are held together in these 

 balls, but it seems probable that they are glued by the adhesive 

 substance from the foot gland. This is supported by the fact 

 that when a young ball is killed and placed in alcohol it breaks 

 up, as if the adhesive material had been dissolved. The animals 

 when in this ball are always nervously jerking back and forth. 

 The ball swims freely by means of the currents from the trochal 

 discs. If, for any reason, more of the individuals turn their 

 trochal discs in a certain direction, the whole colony will move 

 in that direction. That the movement of the ball is due to the 

 summation of currents is shown by mixing some india ink with 

 the water. It is then seen that the ball always moves in the 

 direction from whence the stronger current comes. The ball 

 swims with a revolving motion but is nevertheless able to move 

 with considerable precision in a certain direction. A most im- 

 portant characteristic of these young balls is the fact that they 

 react positively to light. One always finds them on the side of 

 the dish next to the window. If they are placed in a dish in the 

 sunlight, the balls congregate on the side towards the sun. If 

 this region is shaded they at once leave it and move towards a 

 part more strongly lighted. A considerable number of experi- 

 ments showed that this reaction is very marked and constant. 



The time which the animals spend in this swimming ball before 

 settling down to form a permanent colony varies considerably, 

 depending primarily upon the illumination. So far as I have ob- 

 served, the colonies usually hatch out during the morning hours, 

 the young ball breaking away about noon or afterwards. In no 

 case have I seen them, under natural conditions, form a colony 

 until evening or after sundown. As long as the dish is lighted, 

 even by diffused light, the young rotifers continue to swim back 

 and forth along the lighted side of the dish, striving apparently 



