143 
figure of the entire female given by that author, the spiniform projections of the last thoracic 
segment are shewn to be nearly straight, and extend to the end of the second abdominal segment. 
This species has been recorded from the Atlantic and Pacific Oceans and from the 
Mediterranean. 
2. Arvetellus aculeatus (PD. Seott). Plate XLIV, figs. 4—7. 
Rhincalanus aculeatus T. Scott, 1893, p. 31, pl. Il, figs. 11—22. 
Arvetellus setosus Giesbrecht & Schmeil, 1898 (pars), p. 124. 
One adult female identical with the above species was obtained from the plankton 
collected with the HENsEN vertical net at Station 148, 1000 metres to the surface. 
Length of female 4,56 mm. 
This species was originally described by my father from a rather immature male as 
Rhincalanus aculeatus. It was afterwards included under Arzetellus setosus by GriEspRECHT 
and SCHMEIL. 
The discovery of the adult female shews that it is apparently quite distinct from the 
type of the genus. The frontal spine is very strong, much elongated and directed almost straight 
in front (Plate XLIV, fig. 5). The spiniform projections of the last thoracic segment are 
slightly asymmetrical and rather divergent. The spine of the right side is slightly longer than 
that of the left, but it does not reach the distal end of the genital segment when the animal 
is viewed from above. The fifth pair of feet of the female is asymmetrical. The right foot is 
obviously longer than the left. The second free joint of each foot is very broad. The distal 
end of the inner margin is considerably inflated. The apical spine on the right foot is as long 
as the joint. The joint bears an indication near the middle, of an outer marginal spine. The 
apical spine on the left foot is decidedly longer than the joint (Plate XLIV, fig. 7). 
This Arzetel/us has hitherto only been known from the Gulf of Guinea. 
3. Arvetellus simplex Sars. Plate XLIV, figs. 14—18. 
Artetellus simplex Sars, 1905 (4), p. 22. 
Artetellus major Esterly, 1906, p. 74, pls. 9, 11, 12 & 13. 
One male agreeing very well with the preliminary description of the above species 
given by Sars, was found in the plankton collected with the HENsEN vertical net at Station 
I14I, 1500 metres to the surface. 
Length of ‘Siboga’ male 6,2 mm. 
The forehead appears only very slightly produced, when viewed from above. The distal 
ends of the last thoracic segment are boldly rounded. A feeble indication of a projection is 
visible near the junction of the cephalothorax with the first abdominal segment (Plate XLIV, 
fig. 14). The anal segment and furcal joints are decidedly longer than in the previous species. 
The furcal joints are nearly three times longer than broad. The fifth pair has a general resem- 
blance to the fifth pair of the male of Avzete/lus setosus, but the outer margin of the apical 
143 
