HYDROID^ OF THE WOODS HOLK KKGHXY. 



349 



(,'itiiiisniii,: Female gonangia very large and abrupt ly tiuticated above; male gonangia much 

 smaller and more oval, but with no neck; sexual products forming planuhe before leaving gonangia. 



Disli'iliiilinii. Very abundant on floating seaweed, and rock* and timbers in shallow water. One of 

 the most abundant species at Woods Hole. 



OBELIA. 



TrnpJioxniiie. Colony branched, stem simple or fascicled. IIydrothec;e campanulate, margin 

 even, or toothed. 



ilniitiMiiiii: Gonangia borne in axils of pedicels, usually oblong ovate, with terminal aperture 

 usually surrounded with a collar or short neck. Mrdus;c with disk-shaped umbrella, 4 radial canals, 

 more than 8 marginal tentacles, 8 lithocysta 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 

 by the trophosome alone. 



35. Obeliajlabe/lata Hincks. 



36. Olelia commissuralie McCr. 



Key to the species of Obelia 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. Hydrothecse triangular. Pedicels usually with more than 6 annulations 0. fabeUata. 



b'. Hydrothecse deeper, subtriangular. Pedicels often with more than 6 annulations 0, commiwuraJis, 



a'. Colony irregularly branched; branches erect, often themselves branched. Hydrothecse large, very 



deeply campanulate 0. 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 O. geniculata. 



A'. Hydrothecal margin toothed. Stem fascicled. 

 a. Teeth bimucronate, or castellated. 



6. Hydrothecse triangular, without vertical lines 0. gelatinosa. 



b'. Hydrothecee deep, ornamented with vertical lines. 



c. Hydrothecse deeply tubular. Pedicels with 6 to 15 annulations 0. bicuspidata. 



c'. Hydrothecse shorter. Pedicels with 3 to 6 annulations 0. bidentata. 



a'. Teeth forming a series of exceedingly shallow undulations around the hydrothecal margin 0. longissima- 



