LEPTOMEDUS.E — OBELIA. 



243 



or 6 canals. Yet medusx of Obeha with 5 canals have not succeeded in perpetuating a new 

 5-rayed species. 



Herbst, 1904, Archiv. Entwick.-Mech., Bd. 17, pp. 306-520, finds that medusae of Obelia 

 cease to pulsate in solutions lacking sodium, calcium, or potassium, but containing all of 

 the remaining constituents of sea-water. Pulsation returns, however, when the medusae are 

 replaced in normal sea-water. 



When young the medusae of Obelia commonly swim with bell everted, but this habit 

 becomes less frequent, or is abandoned, in the adult. The medusae of the several "species" 

 are so similar each to each that they can not be distinguished apart. For this reason I have 

 not included descriptions of Obelia multicia Browne, 1902 (Annals and Mag. Nat. Hist., 

 ser. 7, vol. 9, p. 281), from the Falkland Islands. This medusa is said to have heart-shaped 

 tentacle-bulbs, but this description might apply to nearly any one of the known forms of 

 Obeha medusae in certain states of contraction. 



General Characters of the So-called Species of Obelia — Continued. 



♦Distinguished by having gonotheca? arising directly from hydrorhiza. 



■f-Torrey, 1909, describes 0. purpurea from California, with purple tentacle-bulbs (see Appendix). 



