142 
hooked, spiniform projection of the forehead when viewed from the side. It is not always 
possible, however, to make out the hooked projection of the forehead. It is easily telescoped 
and some care is necessary in identification. 
- Haloptilus spiniceps has apparently hitherto only been recorded from the Mediterranean 
and from the Atlantic Ocean. 
16. Family ARIETELLIDAE. 
Genus Arietellus Giesbrecht, 1893. 
The members of this genus can be recognised by the rather tumid body, and by the 
characters of the fifth pair of feet of both sexes. 
The female fifth pair is represented by two free joints attached to a basal part. The 
male fifth pair is well developed and prehensile. The exopodite of each foot is three-jointed 
and asymmetrical. The last joint of the right exopodite is lamelliform, and the apex is rounded. 
The last joint of the left exopodite bears two apical claw-like spines. The endopodite of each 
foot is lamelliform, asymmetrical and apparently two-jointed. The basiopodite is two-jointed. 
The characters that separate the various members of the genus depend almost entirely 
on the form of the body, the length of the frontal spine, the shape of the last thoracic segment, 
and the length of the furcal joints. Obvious differences can be noticed, however, in the structure 
of the fifth pair of feet of both sexes. 
Three species were obtained from the plankton collected by the ‘Siboga’ in the Malay 
Archipelago. 
1. Arzetellus setosus Giesbrecht. Plate XLIV, figs. 8—13. 
Artetellus setosus Giesbrecht, 1893, p. 415, pls. 29 & 39. 
Arietellus setosus Giesbrecht & Schmeil, 1898, p. 124. 
Arzetellus setosus Sars, 1905 (0), p. 5. 
Arietellus setosus Esterly, 1905, p. 189, fig. 42. 
Arietellus setosus van Breemen, 1908, p. 139, fig. 158. 
One adult and one immature male belonging to this species, were found in the plankton 
collected with the HENsEN vertical net at the following stations. 
Stat. 141 (1500 metres to surface). — Stat. 203 (1500 metres to surface). 
Length of adult male 4 mm. 
Arvetellus setosus can readily be separated from the other members of the genus by 
the short and slightly hooked frontal spine, and by the strong spiniform projections of the last 
thoracic segment. The apical spines on the exopodite of the left fifth foot of the male are of 
moderate length. The outer one is longer than the third joint. It has a distinct angular bend 
near the middle. Esrrrty’s figure of the female fifth foot, shews that the second free joint is 
nearly as long as the first joint. The terminal spine is as long as the second joint. In the 
142 
