HETEROGENY AND VARIATION IN COPEPODA. 159 



I then set aside six Cyclops with fourteen-jointed antennae, 

 giving them clean hydrant water containing but little food and 

 some fresh-water plants. At the time of -their separation two 

 had fourteen segments only in the left antenna, while the right 

 antenna of each contained a dividing segment, the tenth from 

 the base of the antenna, or the fourth from the distal end. Two 

 weeks later the division of the segment was still incomplete, 

 showing that in this case at least, the formation of partition walls 

 is not very rapid. The bodies looked lighter and clearer than 

 before, and I examined them again to see if any changes had 

 taken place, but none had occurred. 



In his explanatory notes accompanying Plate XXXIV. 1 which 

 shows the species-characteristics of C. parcus (Herrick), Herrick 

 shows "caudal stylets of an elongate form," in Fig. 3, with 

 which my own drawings agree perfectly. It is quite possible 

 that the elongated distal segment of the fifth foot may be a mere 

 variation correlated with the elongation of the caudal stylets in 

 Herrick's ' elongated form ' of C. parats which he suggests " is 

 to be regarded as a post-imago." 



A single characteristic which Herrick describes for C. pul- 

 chcllus, but of which no mention is made in the characterization 

 of C. parcns, to my knowledge, is the presence of serrations on 

 the distal margins (ventral and lateral) of the last abdominal 

 segment, while the remaining margins of the abdominal segments 

 are free from such markings. All of the individuals of the three 

 groups i. c., of the thirteen-, the fourteen- and the seventeen- 

 jointed antennae agree with C. pulclicllus in having these ser- 

 rations, while Groups I. and II. also agree in having " two rather 

 long setae" which are not at all or only slightly plumose on the 

 terminal segment of the fifth foot. But they all differ from C. 

 pulclicllus in not having the basal joint of the fifth foot longer 

 than \vide ; the basal joint is unequivocally wider than it is long, 

 and in this respect agrees with C. parcus. 



Although the armature of the appendages is very constant in 

 the Cyclopidae, it is quite common to meet with similarly placed 

 spines and setae of different lengths. A notable instance of this 



1 " Copepoda, Cladocera and Ostracoda of Minnesota," Zoological Series, II., 

 1895, of the Geological and Natural History Survey of Minnesota. 



