COLD SPRING HARBOR MONOGRAPHS. VII. 



Armature Formula of C. brevispinosus " parcus-stage." 



Foot No. 



Ramus, Outer or Inner.. 



I 



Outer Spines 

 Apical Spines 

 Apical Setie. . 

 Inner SetiK . . 



II 



III 



IV 



The fifth foot is shown in Fig. 9, Plate VII. The labrum and 

 receptaculum agree with corresponding organs of C. brevispinosus. 



For a long time I sought to link this Cyclops with C. parens 

 because of the general resemblance, but more careful study showed 

 the distinctness of the two forms. Some light was thrown on its 

 true relationship, by finding, occasionally, a transitional stage, 

 in which the adult conditions were not fully reached. For example, 

 two cases of C. brevispinosus occurred in which the fourth swim- 

 ming feet were in the "parcus-condition," i. e., having tivo outer 

 spines on the distal segment of the outer ramus, while the remaining 

 feet were thoroughly typical of C. brevispinosus, i. e., having three 

 outer spines on the distal segment of the outer ramus. The arma- 

 ture was as follows : 



Foot No. 



Ramus, Outer or Inner. 



Outer Spines 

 Apical Spines 

 Apical Setie. . 

 Inner Setre . . 



II 



III 



1 



12 



3 



IV 



^seta. ^sejQsg spine. 



The variation is seen in the fourth swimming foot where two 

 outer spines appear in the place of the usual three outer spines of 

 C. brevispinosus. In one case, both right and left rami were alike 



