PliOSPlIORKSCENCE. 145 



Avhilc riackel lias foiiiul similar hairs arranged in three verticils upon tlie knob-like extremity of 

 tile tentacle in another medusa, Tlliopalonema imbUicafiim. It is probal)le that in attributing to 

 these various structures a specially developed sense of louch, tlieir true function has been assigned 

 to them ; but it must, at tlu^ same time, be admitted that we have no direct evidence of their real 

 purport in the economy of the animal. 



(). PhospltoreHCcnce. 



Among the most reniarkal)le faculties possessed by the Hyduoida is the power with which 

 many of them are endowed of emitting light — a power which, like contractility and sensation, 

 ought, perhaps, to find its place among the functions of irritability, as it appears to be always 

 manifested in obedience to the action of a stimulus. 



If a healthy colony of Oldia dicliotoma, for example, be irritated in the dark by being 

 roughly touched, a beautiful pale-white light will be seen for an instant to flash along the branches. 

 Here the power of emitting light would seem to reside exclusively in the trophosome, and I have 

 sought for it in vain in the free planoblasts of this hydroid, where one might, a jjriori, expect to 

 find it. 



On the other hand, in species of Thanmuntias and allied forms which I have met with 

 swimming in the open sea, and which are almost certainly planoblasts liberated from some tropho- 

 some as yet unknowai, the luminosity was very striking and beautiful. In these medusae, when 

 an individual confined in one of my jars was touched in the dark, the whole umbrella-margin 

 became instantly lighted up by a multitude of luminous points. The luminosity was entirely con- 

 fined to the margin of the umbrella, and, indeed, might be seen to have its exclusive seat in the 

 bulbous bases of the marginal tentacles.^ These phosphorescent hydroid planoblasts must be 

 regarded as one of the chief sources of the luminosity of the sea. 



In some of the most brilliantly phosphorescent hydroid trophosomes, the luminosity presents 

 a singular intermittence, the light appearing, when the hydroid is touched, to palpitate in a very 

 beautiful way over the surface. Its duration, however, is very transitory; within a few seconds 

 it will have entirely vanished, and will then need a repetition of the stimulus to call it forth. 

 After a few such repetitions the power seems to exhaust itself, and rest for some time is needed 

 before it can be again excited. 



The vapour of alcohol exerts a very marked influence on the emission of the light. On 

 exposing a campannlarian trophosome to alcoholic vapour, given off at a temperature of about 

 70° F., I was surprised to find that not only was a brilliant luminosity called forth, but that the 

 light had accpiired a persistence very different from its usual transitory manifestation. It con- 

 tinued, indeed, for about five minutes while held over the vapour, after which, though still exposed 

 to the vapour, it gradually faded away. 



It must be here borne in mind, however, that the action of the stimulus in the experiment 



' Busk ('Trans. ]\Iic. Soc. Lond.,' vol. iii, p. 22) has described the luminosity of a Thaumantias- 

 like medusa, in which lie also fouud the seat of the light to be confined to the raaigiual tentacular 

 bulbs. 



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