NO. 6.] ACCOUNT OF THE SPECIES. 37 



slightly protuberant below. Caudal rami scarcely twice as long as they are 

 broad, and obliquely rounded at the end, 3 of the marginal setae issuing 

 from the tip, 2 from the outer edge, middle apical seta almost twice the 

 length of the tail. Anterior antennae slightly exceeding in length the anterior 

 division of the body, the 8th articulation about the length of the 2nd, last one 

 very small. The 3 posterior pairs of natatory legs with the outer ramus 

 very large, and having the terminal spine coarsely denticulate outside. Last 

 pair of legs with the 2nd joint rather short, but distinctly defined, inner spine 

 of last joint twice as long as the apical one, and minutely denticulate outside. 

 Length of body, exclusive of the caudal setae, about 5 mm. 



Male. Anterior division of body less regularly navicular, being abruptly 

 contracted in front, with the cephalic crest obsolete. Last pedigerous segment 

 very small, with the lateral parts somewhat extant. Tail exceeding half the 

 length of the anterior division, 1st segment quite short, 2nd very large, fully 

 as long as the 2 succeeding ones combined; caudal rami mobile, and, as a 

 rule, spread out to each side, marginal setae less fully developed than in 

 female. Anterior antennae comparatively shorter than in female, with the 

 outer half exceedingly slender, proximal part with the joints partly lamellarly 

 expanded, and clothed in front with very large sensory appendages. Posterior 

 antennae and mandibular palps very powerfully developed; oral parts other- 

 wise very much reduced. Last pair of legs about the length of the natatory 

 legs, and rather complicated in structure, 1st basal joint of both legs coalesced, 

 2nd basal joint of left leg simple cylindric, that of right leg much shorter, 

 but very broad, rami of both legs slender, styliform, the outer one 3-artic- 

 ulate, the inner uniarticulate. Length of body 4Va mm. 



Remarks. The sexual dimorphism of this Calanoid is so very great, 

 that it was only after some time that I could convince myself that the 

 remarkable form figured in PI. IX is in reality the adult male of that repre- 

 sented in PI. VII and VIII. At a younger stage, on the other hand, the male 

 resembles the female very closely, and it is accordingly only in the fully adult, 

 or sexually mature state that this remarkable transformation takes place. It 

 would appear from the greatly reduced oral parts, that the existence of such 

 transformed male individuals is restricted to a very short period, and indeed, 

 only a few specimens were found, whereas females and young males occurred 

 very plentifully in the samples examined. 



