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 



FIG. 57. a, b, c, d, Pyrocystia nocliluca; 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- 



