156 Nerve Structure and Force. [April, 



might be, a filiform tentacle, equal in length to about the diameter 

 of the body, depends from it, and exhibits slow wavy motions when 

 the creature is in full activity. The body is composed of a struc- 

 tureless membrane ; beneath this there is a layer of granules, or 

 rather, a gelatinous membrane, through whose substance minute 

 granules are scattered without any definite arrangement. From 

 hence arises a network of very delicate fibres. There is an oral 

 aperture, a sort of half oval, with a straight edge anteriorly, and a 

 deeply curved outline posteriorly. From the bottom of the oral cavity 

 a very delicate filament is occasionally protruded, which exhibits 

 a rapid undulating motion, and is then suddenly withdrawn ; doubt- 

 less, as suggested by Krohn, who first discovered it, the function is 

 to sweep nutritive matter into the oral cavity. Close to the right 

 extremity of the anterior oral margin, is a horny-looking S- shaped 

 ridge, named by Huxley, a tooth, 1-7000 inch high. The oral 

 aperture leads to the granular mass of the alimentary cavity, from 

 which the fibres and the fibrils radiate. 



"What purpose now does the luminosity of Noctiluca serve? 

 Passing down the Mediterranean in 1854 I was struck with its 

 brightness and universality on most nights. Is it a voluntary act 

 on the part of the animal, or is it determined only by reflex laws ? 

 In the fire-fly (Lampyris Noctiluca) it serves for the attraction of 

 the sexes, and can be extinguished and renewed as the animal 

 pleases. In Noctiluca miliaris, its purpose must be for the attrac- 

 tion of its minute prey : the oral aperture, its tongue and teeth all 

 point to the fact that the animal is carnivorous, and serves to keep 

 down those still more minute animal forms, which might otherwise 

 render the water in which they five and generate, impure. But no 

 distinct nervous system has here been traced. 



Another class of marine animals, also carnivorous, the Actinias, 

 live among rocks and broken surfaces, where they perform a similar 

 purifying function, by the destruction of animals of larger form 

 and of apparently much greater power than they themselves possess. 

 But in these, as in the preceding, no trace of nerve matter can 

 be clearly found, and yet they exhibit a power of selection which 

 gives evidence of sense. If a young actinia be put within the ten- 

 tacles of a larger individual, that fatal grasp is withheld, while a 

 young crab or prawn is promptly seized and devoured. Its migra' 

 tions, though slow, have usually some definite purpose : it avoids 

 bright light, preferring the shelter of a piece of rock ; it requires air, 

 and will hang, tentacles downward, for hours from the surface of the 

 water, floating by means of an air-bladder of its own formation. 



Near the oral aperture we often see a series of bright spots, 

 called Ocelli ; they are not always present, and are subject to change 

 of colour — are brighter at some times than at others. They have 

 been considered by some as organs of vision placed on nerve ganglia, 



