146 PHYSIOLOGY. 



witli the alcohol is continuous, while the mechanical stimulus exerts itself only for a moment. A 

 stimulus presenting a closer parallelism with that of the alcohol is afforded by contact with 

 atmospheric air. I have always found that the hydroid, when sudilenly removed from the water 

 into the air, becomes brilliantly luminous at the moment of the change of medium ; the luminosity 

 here, however, as in the case of simple mechanical stimulus, lasts only for a few seconds, notwith- 

 standing the continuous action of the atmosphere on the animal. After its disappearance the 

 vapour of alcohol will again call it forth, accompanied by the persistence which characterises the 

 application of this stimulus. 



Many hydroids, however, are destitute of phosphorescence. It does not exist in the fresh- 

 water Hydra, and I have never witnessed it in any gynmoblastic fomi ; while among the 

 CalijptobJastea it would seem that only some are endowed with it. Observations, however, are 

 still wanting on this point, and phosphorescence will doubtless yet l)c found in species in which it 

 is not at present known to exist. 



Of the immediate source of the phosphorescence we know scarcely anything, and though in 

 some other phosphorescent animals it would seem to reside in a special luminous secretion we 

 have no evidence of such a secretion in the Hydroida. Its dependence here on the operation 

 of a stimulus would remove it from simple physical luminosity, such as may result from a process 

 of slow combustion or from insolation. It may, it is true, be asserted that the immediate result 

 of the stimulus is to excite the formation of a luminous secretion, but in the absence of all evidence 

 of any such secretion this explanation cannot be accepted. The luminosity would here seem 

 rather to be, like electricity in other cases, the direct accompaniment of certain vital actions.' 



7. Reproduction. 



The reproductive faculty, whose exercise gives rise to the calling into existence of a new 

 being, belongs properly to the department of Physiology, while development, or the successive 

 changes of form which this being undergoes, is a subject of Morphology, and has been already 

 treated of in its proper place under that head. 



Reproduction in the livDROiDA may be either sexual or non-sexual; the sexual showing 

 itself in the production and fertilization of the ovum, the non-sexual in the production of buds 

 and in fission. 



' Some observatious which I have made on the lumiuosity of Beroe have brought out the some- 

 what unexpected result that the faculty of emitting light is not possessed by these animals at all 

 times during the twenty-four hours. It does not exist in the presence of daylight, and a previous 

 seclusion of the animal for some time in darlcness is always necessary for its manifestation (see ' Proc. 

 Roy. Soc. Edinb.,' 1863, p. 519). I have no reason to believe that this is the case with the Hydroida, 

 though I have made no observations which can here be regarded as decisive. It certainly is not the 

 case with NoctUuca, one of the most vividly luminous of animals, and one well fitted for observation. 



