306 PHYSIOLOGICAL CONTROL IN ANIMALS 



Teleosts 



From the viewpoint of the evolution of the vertebrate nervous 

 system, one of the most intriguing aspects of photogeny in fishes is 

 the character of nervous regulation. Some pelagic teleosts, notably 

 myctophids, can emit very brief flashes with short latent periods. 

 Receptors are tactile and visual. The innervation of the photophores 

 has been worked out in three forms, viz., Argijropelecus, Cyclothone, 

 and Lampanyctus (Handrick, 1901; Gierse, 1904; Ray, 1950). None of 

 these workers traced the neural pathways involved, but from the pat- 

 tern of distribution of efferent nerves I hazarded the suggestion that 

 they might be autonomic (Nicol, 1952a). 



Earlier work on the midshipman Porichthys showed that the photo- 

 phores could be excited by electrical stimulation of the whole fish, 

 and by injecting adrenaline or pituitrin. Since nerve fibers to the 

 photophores were little in evidence, it was concluded that regulation 

 of luminescence was hormonal (Greene, 1899; Greene and Greene, 

 1924). I find that electrical stimulation of the spinal cord (condenser 

 shocks) causes all the photophores to luminesce, both those of the 

 head and trunk. Transection of the cord, anterior to the electrodes, 

 and ligaturing the heart fail to prevent the response. 



The latent period of the response under electrical stimulation is 

 only 7-10 seconds, a period far too short to be explained by hormonal 

 excitation. The circulation time of fish is not known, but is likely to 

 exceed 10 seconds. Moreover, when adrenaline is injected directly 

 into the heart (0.1 mg in a 1.5-kg fish), it takes 2 minutes for the 

 luminescent response to appear, and this at a concentration which 

 can be considered highly potent. It is apparent that the luminescent 

 responses of Porichthys and certain other teleosts appear far too 

 quickly to be governed primarily by endocrine mechanisms. 



These observations can be explained most easily by postulating that 

 the photophores of Porichthys are innervated by the sympathetic 

 nervous system. The ability to respond in the absence of circulation 

 shows that the endocrine system is not essential to the response. 

 Teleosts generally possess well-defined sympathetic trunks which 

 connect with cranial nerves, including the facial (Young, 1931). The 

 latter provide avenues for peripheral distribution of sympathetic fibers 



