34 COLD SPRING HARBOR MONOGRAPHS. VU. 



Summary. 



Distribution. — A survey of the Cyclops from the Long Island 

 waters shows that they agree in general with the forms from the 

 western lakes and indicates a rather wide distribution of these 

 species. 



Cyclops appear in swarms in certain localities when conditions are 

 favorable and after a period of activity die out, leaving but few 

 representatives. Because of this, forms which are most abundant 

 at one season may become very scarce at another. It is probable, 

 therefore, that the species or varieties reported as sporadic in a 

 locality, such as C. hicolor, may have a season of activity when 

 they are far more numerous than might be inferred from the find- 

 ing of a single individual. 



Variations in Cyclops. — The differences that occur between the 

 western species of Cyclops and those encountered in the waters of 

 Long Island, such as the differences in the number of spines and 

 set?g in the armatures of the swimming feet ; differences in the 

 proportions of the body, etc., are evidently of a varietal nature. 



Although the armatures of stable types are relatively constant, 

 spines and setae are interchangeable as is seen in the occasional 

 substitution of a spine for a seta and in the occurrence of half 

 spines in which one side of a spine becomes very strongly plumose, 

 while on the opposite side are equally developed serrations. 



These half spines may evidently occur anywhere, but it is sig- 

 nificant that they usually occupy either apical positions or lateral 

 positions which approach the apex, where spines and setae occur 

 side by side. This form of variation has been found in widely 

 different types such as C. atcr, C. signatns and C. viridis. 



An interesting case of variation occurs in the armature of C. 

 signatns annnlicornis, where in the fourth pair of swimming feet 

 the distal seta on the inner margin of the inner ramus has suffered 

 a reduction so that it may be present as a long seta more slender 

 than the other setse, or it may appear as a very short slender struc- 

 ture without barbs, or, as is often the case, it may be wholly want- 

 ing, its site then being marked by a cluster of very diminutive 

 hairs, as shown in Plate IV., Fig. 4. 



An apparent variation in the spines of Cyclops is often due to a 

 retardation of growth in the posterior appendages. Retarded 



