768 MICKOSCOPIC FOKMS OP ANIMAL LIFE 



and even twelve or more are occasionally discernible* The place of 

 each in the body is constantly being changed by the contractions of 

 the protoplasmic substance, these in the first place carrying it from 

 the centre towards the periphery of the body, and then carrying it 

 back to the central mass, into whose substance it seems to be 

 fused as soon as it has discharged any indigestible material it may 

 have contained, which is got rid of through the mouth. Every part 

 of the protoplasmic reticulation is in a state of incessant change. 

 which serves to distribute the nutrient material that finds its way 

 into it through the walls of the digestive vesicles; but no regular 

 cyclosis (like that of plants) can be observed in it. Besides the 

 'digestive vesicles,' vacuoles filled with clear fluid may be distin- 

 guished, alike in the central protoplasmic mass, and in its extensions 

 as is shown in the centre of fig. 587. There is no contractile vesicle. 

 The peculiar 'tentacle ' of Noctlluca 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 



FIG. 589. -Pair of digestive vesicles of NoctUnca lying in course of exten- 

 sion of central protoplasmic mass, a, to form peripheral reticulation, 

 6, and containing remains of Algte. (Magnified 480 diameters.) 



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 straightens 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 forming 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. 



The extreme transparence of Noctiluca renders it a particularly 

 favourable subject for the study of the phenomena of phosphorescence. 

 When the surface of the sea is rendered luminous by the general 

 diffusion of Noctilucce, they may be obtained by the tow-net in un- 

 limited quantities ; and when transferred into a jar of sea-water, 

 they soon rise to the surface, where they forma thick stratum. The 

 slightest agitation of the jar in the dark causes an instant emission of 



