12 
TH. MORTENSEN. 
(Schwed. Südpolar-Exp. 
The genital pinnules continue very far out on the arms (PI. II, Fig. 9), may even 
continue nearly to the end of the arm, only some 6 — 7 pinnules at the end of the 
arm being devoid of genital organs and without the characteristic widening of the 
basal joints. Often pinnules without this widening occur between the genital pinnules; 
these are regenerated pinnules, as may be concluded with certainty from the fact that 
they may be found in different stages of growth. 
The very characteristic appearance of the female genital pinnules (PI. II, Figs. 
6 — 7) is due to the enormous widening of the 3rd and 4th joint, forming a brooding 
chamber. The widening is mainly on the aboral side of the joint. In the lower part 
of the arm also the fifth pinnule joint is slightly widened; farther out on the arm 
Fig 9. Basal part of arm of Isometra 
vivipara , side view, showing oral pin- 
nules and an extra fourth pinnule. 9 /i. 
Fig. 10. Male genital Fig. 11. Distal pinnule of 
pinnule of Isometra Isometra vivipara. IX /i. A 
vivipara. 8 /i. pair of joints represented in 
a larger scale; 3 %. 
the widening diminishes, first the 5th, then also the 4th joint assuming the normal 
appearance, the widening of the third joint alone remaining. A corresponding devel- 
opment of the genital pinnules takes place with the growth of the specimens, young- 
specimens having only the third joint widened on the genital pinnules in the lower 
part of the arm, the more distal pinnules showing no widening at all. 
In the male specimens (PI. I, Fig. 6) the genital pinnules have also a distinct 
widening, though, as might be expected, not so large as in the female. The widening 
begins on the third joint, which is the widest, and continues till about the middle of 
the pinnule, gradually diminishing. The widening is almost equally developed on 
both sides of the joints (Fig. 10). 
