Page:The American Cyclopædia (1879) Volume I.djvu/95

 ACTINIA ACTINISM 75 here combined with a kind of circulation ; they have no blood, no vessels, no respiration other than that effected by the currents of water in the interior, doubtless accompanied by a change of substance. The surface of the ten- tacles is thickly studded with microscopic vi- bratile cilia in constant motion, causing cur- rents which bring to them their microscopic food, sweeping a space of several inches. Each tentacle is a tube, with longitudinal and circular fibres, by which it can be shortened, lengthened, and moved in all directions. Upon the tentacles are great numbers of microscopic so-called "lasso cells," each containing a long hollow thread coiled spirally within it, which can be suddenly thrust, out, benumbing and arresting shrimps and small fish incautiously venturing too near these innumerable and in- visible threads, and enabling the tentacles to seize and convey them to the central mouth. Similarly armed threads may also be projected from the sides of the body. The eggs are very numerous, being in bunches on the inside of the partitions until ready to be hatched, when they escape through the stomach and mouth, or through the tentacles, into the water, giving Anthea Cereus (Opelet). rise to creatures like themselves, only with fewer tentacles, which are in multiples of five. The young one has only five, one in the line of the mouth and the others in two pairs later- ally ; so that even here there is an indication of bilateral symmetry, with definition of an- terior and posterior regions. The actinia is the type of the single polyp, as distinguished from the compound coral polyps. It preys voraciously on small crabs and mollusks, and when waiting for its victims these arms are expanded like the petals of a flower, and, being tinted with very brilliant colors, they present an elegant appearance. The actinia seizes animals apparently superior in strength and bulk, engulfs them in its sac or stomach, and distending itself to a great degree, digests them rapidly, disgorging the shells and harder parts of the victim when the softer parts have been consumed. Some actiniae are fixed, and others are free. The external tunic of the body pre- sents both longitudinal and transverse muscular fibres, covered, by a layer of skin or mucous membrane. Nervous fibres have also been de- tected, and the sensibility of the animal is ex- treme ; they contract even when a dark cloud passes over them. They may be seen at low water, clustered upon rocks and masses of stone, which they cover, as with flowers. There they remain tenaciously adhering by their base. They are, however, capable of moving from one spot to another ; and in win- ter they seek deeper water, where the changes of temperature do not affect them. The sea anemone is very common on the southern shores of England and on the New England coasts ; and one species (actinia Jordaica), on the shores of the Mediterranean, is esteemed a great delicacy by the Italians. The fringed actinia (metridium), the most common on the N. E. coast of North America, is, in large specimens, about 4 inches high and 3 inches across the expanded disk. They are found of va- rious colors, pink, brown, purple, whitish, and orange, in pools among the rocks, flooded at high tide, and overhung by seaweeds. In an- thea cereus, of the British coast, there is no power of retracting the long tentacles within the body ; the body is of a light chestnut color, and the numerous tentacles usually sea-green tipped with red. It is of about the size of our fringed actinia. See " British Sea Anem- ones," by Philip Henry Gosse (London, 1860), and " Coral and Coral Islands," by James D. Dana (New York, 1872). ACTINISM (Gr. a/cix, a ray of light), the pe- culiar property or force of that portion of the sun's rays which produces the chemical effects shown in photography. That the actinic raya are different from those which produce heat and light was shown as far back as 1842 by Prof. J. "W. Draper of New York, who recognized in them a new principle or force, for which he proposed the name of tithonicity, and for the rays that of tithonic. The name now adopted was given by Mr. E. Hunt of England. It is found that actinism does not exist in the most luminous rays of light, and that these rays ac- tually tend to prevent the peculiar effects of this force upon inorganic matter. The quan- tity of actinism in the sun's rays varies with the time of day and with the seasons. It is intercepted by red, orange, and yellow glass ; hence photographers now use glass of these colors to admit light to their so-called dark rooms. Such glass transmits the solar heat, while blue and violet glass, which transmit lit- tle or nothing of this heat, transmit the actinic rays. The reason of this has been explained by experiments in taking photographs of the solar spectrum ; they proved that no actinism exists in the red, orange, and yellow rays, that it commences feebly in the green, becomes stronger in the blue, and is strongest in the violet ; but what is remarkable, it is also found to extend far beyond the latter color, in the dark space entirely outside the visible spec- trum. In photographing the spectroscopic lines, it is found that this dark space contains scores of them as well as the visible part of the spectrum, and it appears that the only rea- son that we do not see these ultra-actinic rays