BYRNES: CYCi^OPS OF FRESH WATER. 19 



in having the reduced number of spines: in the other case the 

 variation occurred only on one side of the body. 



As the fourth feet develop more slowly than the more anterior 

 ones, in accordance with the method of development of appen- 

 dages in the Crustacea generally, it seemed not improbable that 

 the forms with the three outer spines on the terminal segments 

 of the rami might represent older forms that had passed through 

 the condition with only two outer spines. 



To evidence of relationship taken from variation in the armature 

 of the swimming feet was added that of the succession of the 

 two types in an aquarium kept under comparatively constant con- 

 ditions for nearly two years. 



In February and March of 1904 all the individuals taken from 

 this aquarium were without exception Cyclops of the viridis type, 

 having three external spines. 



Nine months later I again had occasion to search the aquarium 

 for these same forms, but failed to tind a single one. The only 

 forms present were Cyclops in the "parcus-stage" with tzvo outer 

 spines, although the characteristic. spine of C. brevispinosus on the 

 outer margin of the inner ramus of the fourth foot, was present. 



C. brevispinosus was then reared to determine the relationship 

 of these two forms experimentally. In the summer of 1905, steri- 

 lized aquaria were set up at Cold Spring Harbor and stocked, each 

 with female C. brevispinosus. The conditions under which Cyclops 

 can be reared without introducing sources of error are necessarily 

 ones which influence and limit the food supply and consequently 

 affect growth. All of the water used in these experiments was 

 filtered and the food, consisting of decayed leaves and water 

 plants, was boiled. 



These aquaria were examined on the 15th of the following 

 September and a few young forms were found in two different 

 stages. The younger form had the following correlated charac- 

 teristics: nine segments in the antennae and two segments in the 

 rami of the swimming feet. The spines of C. brevispinosus were 

 very distinct in the fourth foot, inner ramus, even at this early 

 stage. This distal joint, which in this younger stage is the second 

 instead of the third segment, is armed as follows: 



