HETEROGENY AND VARIATION IN COPEPODA. 153 



The chief morphological features by which species of Cyclops 

 are recognized are the following : 



1. The number of joints in the antennae. 



2. The number of joints in the rami of the four swimming feet. 



3. The armature of the swimming feet. 



4. The number of joints in the fifth foot, which is rudimentary. 



5. The shape and armature of the segments of the fifth foot. 



6. The structure of the abdomen with the caudal stylets and 

 the armature of the caudal stylets. 



7. The shape of the receptaculum seminis. 



8. The armature of the maxillipeds. 



9. The relation between the length of the antennas and the 

 cephalothorax. 



The characteristics of C. parcus are as follows : 



1. Seventeen-jointed antennae. 



2. Three-jointed rami in the swimming feet. 



3. Armature of the last segment of the swimming feet. 



FIRST FOOT. SECOND FOOT. 



Outer Rainus. Inner Ramus. Outer Ramus. Inner Ramus. 



2 outer spines. I outer seta. 2 outer spines. I outer seta. 



2 apical setae. I apical spine. I apical spine. I apical spine. 



I apical seta. I apical seta. I apical seta. 



2 inner setae. 3 inner setae. 3 inner setse. 3 inner seta;. 



THIRD FOOT. FOURTH FOOT. 



Outer Ramus. Inner Ramus. Outer Ravins, Inner Rainus. 



Like second. 2 outer spines. I outer seta. 



I apical spine. 



I apical seta. 2 apical spines (equal). 



3 inner setae. 2 inner spines. 



4. Two-jointed fifth foot. 



5. The basal joint short and broad with a single seta on the 

 outer margin. 



A long, cylindrical, distal segment with a blunt, inner spine 

 and a long, outer seta, but very slightly plumose. 



6. The abdomen is composed of segments, the first of which 

 is as long as the remaining segments combined. The caudal 

 stylets are long. 



7. The receptaculum seminis is broadly oval. 



8. There are four hairs on the distal segment of the larger 

 branch of the maxillipeds. The second segment has a large, im- 

 movable dactyl with a row of teeth along the edge, and with a 



