BYRNES: CYCLOPS OF FRESH WATER. 35 



growth in the third and fourth feet of C. signatiis coronatus is 

 shown in a young Cyclops with ten antennal segments and two- 

 jointed rami. In this Cyclops the swimming feet have such an 

 armature that should the first and second feet become three-jointed, 

 they would be fully armed as in adults. If, however, the third 

 and fourth feet should become three-jointed, they could not acquire 

 the adult armature without the formation of new hairs which when 

 they first appear are quite short. In such instances the apparent 

 variation in the size of the setae is not real variation in the usual 

 sense of the word, but belongs to an entirely different category of 

 phenomena; — to sequence in the development of organisms that 

 undergo slow metamorphic changes and in which the more posterior 

 structures develop later than corresponding anterior ones. Such 

 instances have been observed in C. signatus, C. Amcricamis Marsh, 

 and in C. brcvispinosus. This marked reduction in the size of the 

 lower spine on the terminal segments of the swimming feet has 

 also been observed in C. serrulatus. It is therefore of general 

 occurrence but is indicative of recent formation rather than of any 

 marked tendency to vary in that particular respect. 



In the development of spines and setse on the swimming feet 

 of Cyclops the more distal structures form first. This explains the 

 absence of the lower seta on the basal segment of Fig. 4, Plate 

 III., where there is no inner seta in a young form having eleven 

 antennal segments. In adults, this basal segment invariably carries 

 a seta. The later development of this seta is also expressed in the 

 adult by its relatively small size when compared with the more 

 distal setae of the same ramus. 



Herrick recognized two sub-species of the species Cyclops vir- 

 idis, JuRiNE (1820). The American subspecies he designated C. 

 American Its, Marsh, and as varieties under the subspecies, he 

 grouped C. viridis, Cragin (1883); C. parens, Herrick (1883); 

 C. hrevicornis, Herrick (1884); C. Americaniis, Marsh (1893), 

 and C. viridis, Turner (1892). The occurrence of so many forms 

 belonging to the same type and concerning whose identity there 

 exists such a wide difference of opinion, is in itself evidence of a 

 wide range of variation. Forbes records having found forms which 

 completely bridge the difference between two extremes of a series 

 which have been named as separate species, i. e., C. bicuspidatis 

 and C. naviis of Herrick. 



