ANIMAL LUMINOUSNESS. 



203 



In elater Ignitus the masses of luminous sub- 

 stance are extremely irregular in their figure; 

 they are situated close to the posterior angles 

 of the corselet, and are more loose in their 

 structure than the same parts in elutor noctilucus. 



The luminous proboscis or snout of the 

 Julgora is hollow, and has a free communica- 

 tion with the external air by a narrow slit 

 situated near the base of the organ. Its cavity 

 is lined with a fine membrane, between which 

 and the outer translucent corneous crust, there 

 is interposed a soft tissue of a pale reddish 

 colour, arranged in lines longitudinally, which 

 is supposed to be the seat of luminousness in 

 this insect. 



In several instances it has been found that 

 the light-giving substance has continued to 

 shine for a considerable time after being re- 

 moved from the body of the insect. In such 

 cases it has been observed that there appeared 

 to be no diminution either in the weight or the 

 bulk of the luminous organs, excepting what 

 was obviously produced by evaporation of the 

 fluids. 



VI. Geographical distribution of luminous 

 animals. In almost all seas, in every latitude 

 from 60 S. to 80 N., have light-giving ani- 

 mals been seen ; but they are more abundant, 

 and shine with greater brilliancy, in the tropical 

 than in the colder climates. In general, it is 

 observed by voyagers, the luminous mollusca 

 and acalephae occur in greatest numbers not far 

 from land ; and that they are particularly plen- 

 tiful in the seas surrounding groups of small 

 islands within the tropics. 



The luminous insects are met with chiefly 

 in the warmer climates of the temperate zones 

 and within the tropics. We are not aware of 

 any having been met with beyond the latitude 

 of 58. 



VII. Theories of animal luminousness. 

 Very numerous have been the theories formed 

 by philosophers with regard to the nature of 

 the luminous matter which produces the phe- 

 nomena now under review. From the facts 

 stated above, it appears that we are not yet in 

 a position to determine with certainty whether 

 or not animal luminousness has its source in 

 the operations of any agent already known. 

 At least it appears to us that facts enough have 

 been accumulated to set aside the assumption 

 of most of the theories hitherto promulgated. 

 The following are some of these. 



1. That light is imbibed from the sun's rays 

 by luminous animals and given out in the 

 dark. (Beccaria, Mayer, &c.) 



2. That the light is owing to a kind of com- 

 bustion maintained by the oxygen of the air. 

 (Spallanzani.) 



3. That light is swallowed with the food, 

 and disengaged by peculiar organs. (Brugna- 

 telli.) 



4. That the light-giving matter is composed 

 of phosphorus and albumen ; and that the 

 variations in the intensity of the light depend 

 on the more or less complete coagulation of the 

 albumen, by some internal means placed under 

 the control of the animal's will. (Macaire.) 



5. That a fluid containing phosphorus is 

 secreted by the luminous organs, and shines 

 on its being exposed to the oxygen of the air 

 introduced by respiration. (Darwin, II. Davy, 

 Heinrich, Treviranus, and Burmeister.) 



6. That the luminous organs concentrate 

 and modify the nervous influence so as to form 

 it into light; so that, according to this theory, 

 animal luminousness is an effect solely of vital 

 power. (Macartney and Todd.) 



7. Tiedemann thus expresses his opinion : 

 " Animal luminousness would seem to depend 

 on a matter, the product of the changes of 

 composition accompanying life, and, to all ap- 

 pearance, secreted from the mass of humours 

 by particular organs. This liquid probably 

 contains phosphorus or an analogous combus- 

 tible substance, which combines with the oxy- 

 gen of the air or of aerated water at a medium 

 temperature, and thus produces the disengage- 

 ment of light. The preparation and secretion 

 of this substance are acts of life, which change, 

 augment, or decrease by the influence of ex- 

 ternal stimulants, whose action on the animals 

 modifies their manifestations of life. But the 

 phosphorescence itself depends on the com- 

 position of the secreted matter and cannot be 

 regarded as a vital act ; because, on certain 

 occasions, it continues for whole days and even 

 after the death of the animal." * 



This opinion seems to coincide pretty nearly 

 with that of Darwin, Heinrich, and others, 

 stated above (5) ; and we must admit that it ap- 

 pears to harmonize the facts better than any of the 

 other theories that have been propounded. But, 

 while it seems satisfactorily to assimilate many 

 of the phenomena to others more familiar to us, 

 and more within the reach of our investigations, 

 and thus appears to furnish future inquirers 

 with a key to the elucidation of what yet re- 

 mains obscure, it is obvious that it leaves some 

 of the phenomena unexplained, and that se- 

 veral of these seem to be quite irreconcilable 

 with the theory of the phosphorescence being 

 essentially dependent on the composition of 

 secreted fluids. In some of the extremely 

 delicate acaleph<, for instance, from which the 

 brightest radiance is so frequently emitted in 

 momentary fitful flashes, it is difficult to con- 

 ceive why, if the light mainly depended on 

 the nature of some matter poured out by cer- 

 tain organs at the instant of the flash, the light 

 should not continue for at least a few seconds. 

 The circumstance of its doing so in some in- 

 stances, and even mixing gradually with the 

 surrounding sea-water, certainly proves that 

 there is such a fluid as this theory supposes in 

 certain animals; but does not remove the dif- 

 ficulty presented by the facts we have al- 

 luded to. 



We feel ourselves constrained by these and 

 other such facts to believe (with Macartney 

 and Todd) that in many, perhaps in all, lumi- 

 nous animals, both terrestrial and marine, the 

 light emitted is the consequence of an evolu- 



* Comparative Physiology, translated by Drs. 

 Gully and Lane, i. 270. 



