10 PLANTS IN PONDFISH CULTURE. 



By reference to Table 2, it is seen that the young bass find in the 

 small Bosmina a favorite natural food. It was selected by them 

 from a varied and abundant zooplankton consisting of cladocerans, 

 copepods, and rotifers. In the smallest fry examined the Bosmina 

 content reached 100 per cent. For example, in the examination sev- 

 eral specimens under the average of 12.2 mm. measured 10 mm. and 



11 mm., and in all cases, the smaller the fry taken from the environ- 

 ment of Volvox, the larger the percentage of Bosmina in the food 

 content, indicating that this is not only the earliest but the pre- 

 ferred natural food of the young bass. 



It was found that Bosmina occupied the upper stratum of water in 

 the open areas as well as the more sheltered weedy portions of the 

 ponds. Their presence is indicated to the naked eye by the appear- 

 ance, as it were, of a fine sprinkling of dust particles continually 

 gyrating in the surface film of water. They occur in the greatest 

 numbers just below the surface, and in this location they are accessi- 

 ble to the fry as they rise to feed. 



The Volvox accumulated also in the upper stratum of water. The 

 pulse of this alga coincided with that of Bosmina, but declined before 

 any distinct diminution of Bosmina was noted. Further observa- 

 tional studies disclosed the direct dependence of Bosmina upon Vol- 

 vox for subsistence. 



Plankton catches from the upper stratum were taken repeatedly, 

 and the feeding habits of Bosmina observed under the compound 

 microscope. It was rarely possible to identify the food once taken 

 into the digestive tract, because in most cases the mandibles grind 

 the food i3articles beyond recognition. Occasionally, however, par- 

 ticles slip by whole, and when these could be seen through the trans- 

 parent body walls the animal was lightly crushed under a cover 

 glass to make the identification more sure. Bits of broken coenobia 

 of Volvox were identified, and these graded into the ground material 

 characteristic of the digestive tract. In most cases the feeding hab- 

 its were observed directly by watching the maneuvers of the living 

 animal. Volvox was in the reproductive stage, and organisms with 

 antherozoids, or sperms, were exceedingly abundant. The Bosmina 

 in their feeding operations attached themselves to a bundle of ripe 

 antherozoids, and by a rapid movement of the legs, characteristic of 

 all cladocerans, winnowecl the sperms within the body walls, from 

 whence they were wafted into the mouth. This continued until the 

 bundle of antherozoids was appreciably diminished, and the Bosmina 

 whirled off to other feeding grounds, in which the constituents were 

 often too small to identify as they were wafted into the body. Vol- 

 vox, however, continued to remain a source of nutriment until its 

 decline. 



This interdependence of the organisms, Bosmina and Volvox, was 

 observed in pond ID also, but the plankton pulse was not manifested 

 by so large a quantity of Bosmina in this pond. 



SCAPHOLEBERIS AND MOUGEOTIA ASSOCIATION. 



It was found that Scapholeherls mucronata, a larger cladoceran, 

 succeeded the Bosmina in the food of the older fry, and investigation 

 followed to determine its food relations among the algse of the ponds. 



