564 THE BIOLOGY OF MARINE ANIMALS 



immediate vicinity, without affecting distant regions. Excitation of the 

 photocytes in these jellyfish is either direct or involves local reflexes. In 

 the scyphomedusan Pelagia gentle stimulation evokes a local spot of light, 

 which spreads under stronger stimulation until the whole bell and tentacles 

 respond with a glow. In the sea-pen Pennatula Panceri showed that 

 mechanical stimulation at any point causes a wave of luminosity to spread 

 over the colony. Similarly, in the sea-pansy Renilla tactile stimulation 

 evokes luminous waves which pass out concentrically over the colony (16, 

 47). 



In ctenophores the tactile receptors involved in luminescence are 

 situated along the rows of ciliary plates or combs. In dark-adapted 

 Mnemiopsis weak stimulation causes local luminescence on a comb-row, 

 whereas strong stimulation calls forth general luminescence from the 

 meridians of the entire animal. 



The nemertine Emplectonema kandai luminesces over much of the body 

 when strongly stimulated. Local tactile stimulation produces a rather 

 restricted light, but on stretching the worm, luminescence spreads all 

 over the body. In the polychaete Chaetopterus the response to tactile 

 stimulation is very localized, and when a particular segment is irritated, 

 light appears only in that region. For example, when a fan or posterior 

 segment is touched, light appears in the notopodia of the segment con- 

 cerned, and on mechanically stimulating the twelfth segment a bright 

 cloud of luminescent secretion is discharged from a pair of large photo- 

 genic glands in that region (Fig. 13.2). In polynoid worms luminescence 

 is confined to the scales, which flash when the animal is mechanically 

 stimulated or injured. 



In Pyrosoma mechanical stimulation — touch, agitation, water currents — 

 causes the appearance of light, and when the stimulus is localized the light 

 appears first in a restricted area and then spreads over the whole colony. 

 Light from another colony, or from some other source, will also evoke 

 luminescence in a distant colony. Normal excitation, consequently, is 

 both photic and mechanical in this form. Certain fish bearing photophores 

 also luminesce when disturbed. Following tactile stimulation luminescence 

 has been observed in the photophores of lantern-fish (Myctophum), in 

 Maurolicus and in the illicium of the angler-fish Cryptosparas. Myctophum 

 was also observed to respond to weak external illumination (a luminescent 

 watch-dial) by flashing. In other species, well endowed with photophores, 

 luminescence may be very difficult to elicit by mechanical stimulation 

 (e.g. Spinax, Porichthys). 



Chemical stimulation has been investigated in a few forms, and the 

 results, although well marked, are not easy to compare or interpret. 

 Hypotonic solutions frequently induce luminescence, and fresh water was 

 often used by early workers to cause animals to glow. In dilute sea water 

 (about 35 %) Noctiluca becomes insensitive to stimulation and a constant 

 glow sets in. In isotonic cane-sugar the response is normal for two hours, 

 and the addition of 10 % sea water to the sugar solution prolongs the normal 



