A species of Peridinium, which is luminous at night, and occasionally covers large portions of the Bay of Bengal with a scarlet coat by day, nearly approaches the character of the unicellular Algæ. Mr. J. H. Carter observed that at first, when these animalcules were in a state of transition, their nearly circular bodies were filled with translucent green matter, closely allied, if not identical with, chlorophyll, which disappeared when the animal approaches its fixed state, and a bright red took its place: the Infusoria were then visible to the naked eye, and the sea became scarlet. The scarlet state only lasts for a few days, for each of these innumerable Infusoria becomes encysted or capsuled, and either floats on the water, or sinks to the bottom and remains motionless. The Euglena sanguinea has a scarlet state analogous to that of the Peridium. It is so minute and versatile that it is difficult to ascertain its true form, which, however, seems to be a spindle shape, with a pointed and blunt round head. In general it is of a rich emerald green, with perfectly clear, colourless extremities; but it sometimes occurs of a deep red, and in such multitudes as to give the water the appearance of blood.[19]
The Noctiluca miliaris, a luminous inhabitant of the ocean, and the most beautiful of the Infusoria, is distinguished by its comparatively gigantic size, and by its brilliant light, which makes the sea shine like streams of silver in the wake of a ship in a warm summer evening, when they come to the surface in countless multitudes. It is a globular animal like a minute soap bubble, consisting of gelatinous matter, with a firmer exterior, and being about the thirtieth of an inch in diameter, it is visible to the naked eye, when a glass in which it is swimming is held to the light. On one side of the globe there is an indentation, from whence a tail of muscular fibre springs striped with transverse rings, which aids the animal in swimming. At the root of the tail lies the mouth, bordered on one side by a hard dentile lip leading into a funnel-shaped throat, from whence a long flickering cilium is protruded, supposed to be connected with respiration. The throat leads into a large cavity in the gelatinous substance, from whence the rudiments of an alimentary canal descend. From the internal surface of the globe sarcode fibres extend through the gelatinous matter, so as to divide it into a number of irregular compartments, in which vacuoles are often seen. They give buoyancy to the animal, and enable it to rise and sink in the water, but seem to disappear when the food is digested. The sarcode fibres constantly change their form and position, and the electric light emitted by a direct exertion of nerve power, which seems to be constant to the naked eye, really consists of momentary scintillations that increase in rapidity and intensity by the dash of an oar or the motion of the waves.