36 THE MICROSCOPE AND ITS REVELATIONS. 



served in it. Besides the 'digestive vesicles,' vacuoles filled with clear 

 fluid may be distinguished, alike in the central protoplasmic mass, and 

 in its extensions, as is shown in the centre of Fig. 297. There is no con- 

 tractile vesicle. 



429. The peculiar 'tentacle' of Noctiluca is a flattened whip-like fila- 

 ment, gradually tapering from its base to its extremity; the two flattened 

 faces being directed respectively towards and away from the oral aperture. 

 When either of its flattened faces is examined, it shows an alternation 

 of light and dark spaces, in every respect resembling those of striated 

 muscular fibre, except that the clear spaces are not subdivided. But 

 when looked-at in profile, it is seen that between the striated band and 

 the aboral surface is a layer of granular protoplasm. The tentacle slowly 

 bends over towards the mouth about five times in a minute, and straight- 

 ens itself still more slowly; the middle portion rising first, while the 

 point approaches the base, .so as to form a sort of loop, which presently 

 straightens. It seems probable that the contraction of the substance f orm- 



FIG. 299. 



Pair of Digestive Vesicles of Noctiluca, lying in a course of extension of central protoplasmic 

 mass a, to form peripheral reticulation b, and containing remains of Algae. Magnified 480 diam- 

 eters. 



ing the dark bands, produces the bending of the filament; whilst, when 

 this relaxes, the filament is straightened again by the elasticity of the 

 granular layer. l 



430. The extreme transparence of Noctiluca renders it a particularly 

 favorable subject for the study of the phenomena of phosphorescence. 

 When the surface of the sea is "rendered luminous by the general diffu- 

 sion of Noctiluca}, they may be obtained by the tow-net in unlimited 

 quantities; and when transferred into ajar of sea-water, they soon rise to 

 the surface, where they form a thick stratum. The slightest agitation 

 of the jar in the dark causes an instant emission of their light, which is 

 of a beautiful greenish tint, and is vivid enough to be perceptible by 

 ordinary lamp-light. This luminosity is but of an instant's duration, 

 and a short rest is required for its renewal. A brilliant, but short- lived 

 display of luminosity, to be followed by its total cessation, may be pro- 

 duced by electric or chemical stimulation. Professor Allman found the 

 addition of a drop of alcohol to the water containing specimens of Nocti- 

 luca, on the stage of the microscope, produce a luminosity strong 

 enough to be visible under a half -inch objective, lasting with full intens- 



1 According to Robin, the ' tentacle' of Noctiluca is derived conjointly from the 

 cell- wall and from its contained protoplasm; being thus differentiated alike from 

 the 'flagellum,' which he regards as an extension of the latter alone, and from 

 a * cilium,' which is an extension of the former. , 



