316 PHYSIOLOGICAL CONTROL IN ANIMALS 



nervous pathways ( metazoans ) . There are no unequivocal instances of 

 hormonal regulation known, although this mode of control is not 

 necessarily improbable. The interesting condition in the colonial 

 tunicate, Fijrosoma, still awaits examination. In this animal a luminous 

 wave progresses over the zooids making up the colony. Panceri ( 1873 ) 

 found a common system of muscles extending between the zooids and 

 suggested that associated nerve fibers might serve for the transmission 

 of excitation affecting the photocytes. Since illumination evokes lumin- 

 escence, it is also possible that the luminescent wave may be propa- 

 gated by this means. 



In the study of any physiological process, one is assisted greatly 

 in the design of suitable experiments by an appreciation of its role 

 in the economy of the animal. It is indeed a curious fact that although 

 certain aspects of luminescence have advanced greatly, notably the 

 biochemistry of the process, we rarely have any clear appreciation of 

 the significance of the luminescent response to the animal. As an 

 example of how knowledge of the purpose of the response can in- 

 fluence the design of experiments, I refer to the studies of Buck 

 (1948) on the role of luminescence in the mating responses of the 

 American firefly Photinus pyralis. 



Studies now available invite speculation on another aspect of 

 luminescence, namely restricted modes of luminescent control. It is 

 probable that neuro-effector control was established initially for mus- 

 cular systems and extended secondarily to other effectors, including 

 luminescent organs. It is not surprising, therefore, to find certain 

 modes of regulation common to muscular and luminescent systems. 

 Not only has luminescence arisen independently in several different 

 groups of animals, but each of the several forms of luminescence has 

 also appeared independently on several occasions, e.g., discharge of 

 luminous secretion in worms, balanoglossids and many other forms; 

 complex photophores in squid, euphausiids and teleosts; opaque, 

 screening lids in squids and teleosts; and different processes of chro- 

 matophore regulation in the same two groups. Within a few Hmited 

 forms of structural patterns, combination and variation of detail 

 provide multiplicity of response mechanisms. In the words of Sir 

 Thomas Browne: "Studious observations may discover more analogies 



