REPORT ON THE HYDROIDA. 
23 
tentacles into the substance of the umbrella, and. the close approximation of every 
otocyst with the base of a tentacle. 
Among the Hydroids of the Challenger one species of Obelia ( Obel-ia geniculata), a 
very common European form, has been identified. 
Obelia geniculata, Linnaeus, sp. (PI. XII. figs. 1, la). 
Sertularia geniculata, Linn., Svst. Nat., 1312. 
Laomedea geniculata , Larnx., Cor. flex., 205. 
Eucope diaphana, Agassiz, Nat. Hist, of U.S., vol. iv. p. 322, pi. xxxiv. figs. 1-9. 
Obelia geniculata, Allman, Ann. and Hag. Nat. Hist., May 1864. 
„ „ Hincks, Brit. Hydroid Zooph., p. 149, pi. xxv. 
Trophosome . — Hydrocaulus consisting of simple or sparingly branched stems, which 
arise from a network of creeping fibres, and attain a height of about an inch. Hydrothecse 
obconical, with entire margin, alternate, supported each on a strongly annulated peduncle, 
which springs from a projecting hook-shaped process of the stem situated immediately 
below the salient angle of every geniculation, and supported by a bracket-like thickening 
of the chitinous perisarc. 
Gonosome. — Gonangia borne on short annulated peduncles which spring from the 
angles between the hydrothecse and the stem, urn-shaped, gradually widening from below 
upwards and terminating distally in a short conical neck, which carries the orifice on its 
summit. 
Locality. — Kerguelen Island ; depth, 20 to 26 fathoms. 
Station 315, Port William, Falkland Islands; lat. 51° 40' S., long. 57° 50' W.; depth, 
5 to 12 fathoms. 
Among the whole of the Campanularian Hydroids there is perhaps not one in which 
the characters of the trophosome are so definite and so easily recognisable as in Obelia 
geniculata. The remarkable development of the chitinous perisarc which occurs on one 
side of every internode, and supports the alternately disposed hydrothecse in the manner 
of a bracket, has not been found in any other species. 
Obelia geniculata is one of the few British Hydroids obtained by the Challenger. 
In the British and European seas it is one of the commonest and most widely distributed 
species, while it has also been obtained on the eastern and western shores of the United 
States, and on the coast of Labrador (Hincks), as well as in the seas round the North Cape. 
The Challenger dredged it from two localities, one in the region of Kerguelen 
Island, and the other in that of the Falkland Islands. Both therefore in nearly the 
same southern latitudes, though separated by about 130° of longitude. The distance 
between these extreme southern localities and the Arctic Ocean in which it has been 
found by Sars, affords one of the most striking examples known of the wide geographical 
distribution of a single species. 
