HYDROIDS OF THE WOODS HOLE REGION. 
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Gonosome. — Female gonangia very large and abruptly truncated above; male gonangia much 
smaller and more oval, but with no neck; sexual products forming planulse before leaving gonangia. 
Distribution. — Very abundant on floating seaweed and on rocks and timbers in shallow water. One 
of the most abundant species at W oods Hole. 
OBELIA. 
Trophosome. — Colony branched, stem simple or fascicled. Hydrothecte campanulate, margin 
even, or toothed. 
Gonosome. — Gonangia borne in axils of pedicels, usually oblong ovate, with terminal aperture 
usually surrounded with a collar or short neck. Medusae with disk-shaped umbrella, 4 radial canals, 
more than 8 marginal tentacles, 8 lithocysts borne on bases of tentacles, and a short manubrium 
without mouth tentacles. 
It is apparently impossible to define this genus so that it can be distinguished from Campanulciria 
by the trophosome alone. 
Key to the species of Obeliu found in the Woods Hole region. 
(A very careful manipulation of the microscope is often necessary before the characters of the 
hydrothecal margin can be definitely determined.) 
A. Hydrothecal margin entire. Stem not fascicled. 
a. Colony a long central stem, giving off subverticillate branches which are themselves palmately branched. 
b. Hydrothecae triangular. Pedicels usually with more than 6 annulations 0. JlabeUata. 
b'. llydrothecae deeper, subtriangular. Pedicels often with more than 6 annulations 0. commissuralis. 
a'. Colony irregularly branched; branches erect, often themselves branched. Hydrotheca; large, very 
deeply campanulate O. dichotoma. 
a". Colony usually consisting of a single geniculate stem, giving off alternate pedicels which are sup- 
ported on broad shoulders of the internodes from which they spring 0. geniculata. 
A'. Hydrothecal margin toothed. Stem fascicled. 
a. Teeth bimucronate, or castellated. 
b. Hydrothecae triangular, without vertical lines 0. gelatinosa. 
b'. I-Iydrothecae deep, ornamented with vertical lines. 
c. Hydrothecae deeply tubular. Pedicels with 6 to 15 annulations 0. bicuspidata. 
c'. Hydrothecae shorter. Pedicels with 3 to 6 annulations 0. bidentata. 
a'. Teeth forming a series of exceedingly shallow undulations around the hydrothecal margin 0. longissima. 
