BYRNES: CYCLOPS OF FRESH WATER. 15 



have three. The three segments are derived from two by a trans- 

 verse division of the distal one. as Fig. 2. Plate XI.. plainly 

 shows. The same figure shows that the armature is coiuplete — 

 all setse and spines being fully formed — before the segmentation 

 of the ramus begins. At the stage of Fig. 2 the armature is 

 as follows : 



Foot No 



Ramus, Outer or Inner. 



Outer Spines 



Apical Spines 



Apical Setce 



Inner Seta; 



^seta. 



II 



III 



IV 



2 

 



Inspection of this armature will show that when the segmentation 

 of the ramus is complete it will have reached its adult condition. 

 The proportions may change somewhat as the distal segment com- 

 pletes its separation, but the armature remains the same. 



Although repeated search has been made for the transitional 

 forms they have very rarely been seen. 



A comparison of C. Aiiicricanns and C. parens shows such a 

 very striking agreement between the two forms in many respects 

 as to suggest the probability of a very close relationship. This 

 suggestion is further strengthened by the occasional finding of an 

 adult Cyclops unmistakably C. Aviericanns, ^'I.vrsii, in which most 

 of the swimming feet agree with C. Americanns in having three 

 outer spines on the terminal segments of the rami, while others 

 are in the condition characteristic of C. parens, Herrick ; i. e., 

 having but txi'o outer spines on the terminal segments of the rami. 

 These forms are rather rare, as might be expected, but they serve 

 to suggest that the two adult forms that live side by side in the same 

 localities are differing forms of the same species. The suggested 

 relation of C. parens and C. Amerieainis is not based whollv on 

 the variations that occur in the armature of the rami, but partly 

 on analogy with a similar case in C. hrevispinosus, where 

 heterogeny has been definitely determined by experiments in the 



