184 SEAWEEDS 
has been observed only in the open ocean, far from 
land. In coastal waters they occur separately. 
The genus Pyrocystis (P. noctiluca and P. fusiformis), 
observed by Dr. John Murray during the Challenger 
Expedition, and subsequently by other naturalists, is 
Fic. 57.—a, b, ¢, d, Pyrocystis noctiluca; e, P. fusiformis highly magnified. 
After J. Murray.) 
a very puzzling one, and, owing to our insufficient 
knowledge of its life-history, very hard to place in a 
satisfactory systematic position. It is a tropical 
form, and to it was attributed the most brilliant dis- 
plays of luminosity of the sea during the Challenger 
Expedition. It bears a certain degree of resem- 
