Animal Luminescence 573 



finally concluded: " On the whole I believe it most probable that 

 the animal secretes a spontaneously inflammable substance. It may 

 be a compound of phosphorus but it is not necessary to assume that 

 it is." 



Panceri (1872) paid particular attention to the waves of light. 

 He pointed out that the colonies, living at depths of 40 to 100 

 meters, must be freshly collected for good results since, o^ving to 

 poor conditions in an aquarium, pennatulids absorb water and swell, 

 sometimes to double their bulk. An English account states: 



With colonies in good condition a stimulus at one point will result in 

 what he called " luminous currents," as if the little polyps took fire one 

 after another, those on one branch before those of the following one, . . . 

 If we operate on the basal extremity of the stalk, we shall have on the 

 stem an ascending luminous current. If the stimulant is on the con- 

 trary, applied to the top of the polypidom, there will be a descending 

 current. Lastly, if the stimulus is applied to the feathered part of the 

 rachis, one will then obtain two divergent currents. If the two extremi- 

 ties of the polypidom are simultaneously excited, one will have two cur- 

 rents convergent, which usually cease after a period of great vivacity at 

 the point where they meet. I have only once seen in a very sensitive 

 Pennatula the two converging currents continue after thus meeting, each 

 one their own way, as if the other did not exist. 



Panceri's results thus differ from Forbes. 



Panceri observed a latent period of 4/5 second before " applica- 

 tion of the excitement and the commencement of the (luminous) 

 current." He then measured the velocity of propagation in both 

 Pennatula phosphora and P. rubra, finding an average value of 5 cm. 

 per second (temperature not specified) which he took pains to point 

 out is 600 times slower than the Helmholtz figure for the velocity 

 of nerve conduction in the sciatic nerve of the frog. Another rather 

 rare pennatulid, Cavernularia pusilla, behaved in the same way. 



Panceri regarded the transmission of a luminous wave as most 

 remarkable. He suspected that nerve fibers were involved but de- 

 cided that further work would be necessary to determine whether 

 the luminous current is actually transmitted along nerves. Since 

 then the nerve net of coelenterates has become well known, and all 

 later writers have been equally intrigued by the propagation of the 

 luminous wave. 



Siphonophores 



Although a Physalia was reported (1819) as luminous by W. G. 

 Tiselius (1 767- 1 857) , who noticed so many luminous animals in 

 his cruise around the world, the identification is questionable. The 



