other dinoflagellates also impart luminescence to the ocean. 

 Pyrocystis, often confused with Noctiluca , is responsible for many exten- 

 sive displays on the high seas. Organisms such as Ceratium , Peridinium , 

 and Gonyaulax cause rather bright surface luminescence. Gonyaulax also 

 is one of the prime causes of "red tide." 



Other organisms responsible for luminous waters include crustaceans, 

 such as ostracods, copepods, and euphausiids. Most of the displays 

 caused by crustaceans are seen in colder waters and only rarely in tropi- 

 cal waters. The light emitted appears to twinkle at a distance because 

 of abrupt flashing by each individual and is usually blue or green. 



Luminescent jellyfishes (Medusae) also cause many displays. Large 

 shining round or oval spots of light may appear in the water and the 

 resvilting luminous sea can be very bright and cover a large area. One 

 of the most spectacular forms is the large luminescent medusa Pelagia 

 noctiluca. When touched lightly the whole surface of the organism 

 starts to liaminesce, first at the point of contact, then spreading out 

 to the umbrella and tentacles. 



Ctenophores (comb jellies) are almost aill luminescent, giving off 

 a greenish glow. Luminescent transparent tunicates such as Salpa or 

 Pyrosoma are responsible for some of the most spectacular displays re- 

 ported by mariners. The latter organism is abundant in warm waters, and 

 the light produced by large concentrations of colonies has been described 

 by Thomson (I877) while a member of the CHALLENGER Expedition. 



"After leaving the Cape Verde Islands and going south in 

 August, between l4 and 22 W and 5 and I7 N, the sea had 

 been every night a perfect blaze of phosphorescence, the un- 

 broken part of the surface appeeired pitch black, but wherever 

 there was the least ripple the whole line broke into a 

 brilliant crest of clear white light. Near the ship the black 

 interspaces predominated, but as the distance increased the 

 glittering ridges looked closer until towards the horizon as 

 far as the eye could reach, they seemed to run together and 

 to melt into one continuous sea of light. The wake of the 

 ship was an avenue of intense brightness. It was easy to 

 read the smallest print sitting at the after -port in my cabin; 

 the bows shed on either side rapidly widening wedges of radi- 

 ance, so vivid as to throw the sails and rigging into distinct 

 lights and shadows. The first night after leaving San lago, 

 the phosphorescence seemed to be chiefly due to large Pyrosoma 

 of which we took many specimens with a tow net, and which 

 glowed in the water with a white light like that from molten 

 iron." 



The slightest touch at one end of a colony of Pyrosoma can cause blue 

 light to advance along the structure, each individual organism lighting 

 in turn, until the whole luminesces brilliantly. Two popular names for 

 this organism are the "fire body" and "fire cylinder." Various colors of 



