COPEPODA 7 



(Canu 1892), but their structure is quite different from the one described; lately I have examined the 

 labrum and labium in two genera of the Harpactoida, in Oncaea and in Oithona, but only in the 

 latter genus found a structure not quite dissimilar to the one described. 



In the nauplius of Calanus I have not yet succeeded in elucidating the structure of these or- 

 gans; in the first postlarval stage of C. finmarchicus the structure is in its main feature like that of 

 the adult one. 



The structure of the mouth organs in the males is, in species in which the oral appendages 

 are not reduced, scarcely different from that of the female, but in other forms, it is more or less 

 reduced (cf. pi. IV fig. 1 e; pi. VI fig. 3 e). 



From the above it seems to me to be evident that the study of these organs in different forms 

 is of no mean importance. 



On postlarval development. 



Most modern authors, who have studied the Plancton-Copepods from the systematic point of view 

 have only paid attention to the adult males and females, even when younger specimens were present; in 

 later years Ob erg has given a description of the larval stages in several species and Kraeeft has paid 

 much attention to the postlarval development in a very interesting paper; Dam as, Paulsen and 

 Stephensen have for single species contributed to our knowledge of the development. It is generally 

 understood that Grobben is the first who has described the nauplius in Calanus finmarchicus and 

 that Gran was the first to give an account of the postnauplial development; both statements are wrong, 

 as Kroyer, as early as the year 1847, followed the development of Calanus finmarchicus from the 

 nauplius to the adult (or more correctly the penultimate stage). In this paper I have not dealt with 

 the larval forms, but I have tried and generally succeeded in determining the postlarval stages; 

 I have named the first postlarval form stage I, and the adult male and female stage VI. About the 

 differences between the different stages in each species, and the reductions in the limbs I refer to the 

 systematic part; the most complete account is found in the description of Cal. finmarchicus. The 

 mouth-organs are generally not reduced in the Stages IV — V; the number of setae in the exopodite 

 of the maxillulae forms an exception to this rule. In the adult female the number is 11, in stage 

 V it is 10, in stage IV it is 9, in stage III it is 8, in stage II it is 7 and in stage I probably 6. 

 The main differences between the stages is found in the number of natatory legs and number of 

 somites of the urosome. 



Nomenclature. 



In the naming of the different appendages I have followed Hansen as well as Giesbrecht, 

 who somewhat later arrived at the same result. Hansen has shown that the antennae, mandibulae 

 and maxillipeds really have threesegmented basipodites. The structure of the maxillulae in Valdiviella 

 insignis shows clearly, that the basipodite of these limbs are threesegmented; the first basal segment 

 is adorned with the L,i I; the second bears the Le and a single Li (II 3 III); the third basal segment 

 which is distinctly articulated to the preceding one, supports the tvo rami. The natatory legs ap- 



