344. The motions of these cilia are regular, incessant, and when in full activity far too rapid to be distinguished by the eye even when assisted by the microscope. They are generally to be perceived only when their motions are comparatively feeble. They produce two effects. In animals capable of progressive motion, they transport the body through the water, while they constantly bring new strata of water into contact with the respiratory surface. In this case they are partly organs of locomotion, and partly organs subservient to respiration. On the other hand, in animals which are not capable of moving from place to place, they create currents by which the respiratory surface is constantly bathed with fresh streams of water. These currents are regular, constant, unceasing. Like some physical phenomena not depending on vitality, it is a continued stream as regular as the motions of rivers from their source to the ocean, or any other movements depending on the established order of things. Dr. Grant, to whom we are indebted for our knowledge of the true nature of these currents, as well as of the instruments by which they are effected, gives the following account of the observation which led to the discovery:—“I put,” says he, “a small branch of the spongia coalita, with some sea water into a watch-glass, under the microscope, and on reflecting the light of a candle through the fluid, I soon perceived that there was some intestine motion in the opaque particles floating through the water. On moving the watch-glass, so as to bring one of the apertures on the side of the sponge fully into view, I beheld, for the first time, the splendid spectacle of this living fountain, vomiting forth from a circular cavity an impetuous torrent of liquid matter, and hurling along in rapid succession opaque masses which it strewed everywhere around. The beauty and novelty of such a scene in the animal kingdom long arrested my attention, but after twenty-five minutes of constant observation, I was obliged to withdraw my eye from fatigue, without having seen the torrent for one instant change its direction, or diminish in the slightest degree the rapidity of its course. I continued to watch the same orifice, at short intervals, for five hours, sometimes observing it for a quarter of an hour at a time, but still the stream rolled on with a constant and equal velocity.” [Illustration: Fig. CXXIX.—_Diagram of the Apparatus of the Circulation and Respiration in the Fish._ 1. Auricle (Single) of the heart. 2. Ventricle (single) of the heart. 3. Trunk of the branchial artery. 4. Division of the branchial artery going to the branchiæ or gills. 5. Leaves of the branchiæ. 6. Branchial veins, which return the blood from the branchiæ, and unite to form. 7. the aorta, by the division of which the aërated blood is carried out to the system.]