Page:Popular Science Monthly Volume 8.djvu/35

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HYDROIDS.
PSM V08 D035 Sertularina cupressina.jpg
Fig. 7.—Sertularina cupressina.
(Natural Size)

anemones, the hydroids reject such portions of their food as they do not assimilate through the mouth. In the fresh-water hydra an orifice has been observed at the lower extremity of the stomach. This, however, does not correspond to the alimentary canal of higher organisms; it is the analogue, in the simple hydra, of the ramifying cavity which permits a free circulation throughout the compound group.

A circulation has been observed in the varieties which possess a horny sheath, which is, however, very different in some respects from the circulation of the blood in higher organisms. The somatic fluid, as it is called, is loaded with granules which, upon microscopic examination, prove to be composed of disintegrated elements of food, of solid colored matter secreted by the walls of the somatic cavity, of cells detached from the living tissue of the animal, and of particles of effete matter. The fluid seems to be more nearly akin to cliyme, or chyle, than it is to blood. There is perpetual motion in the somatic fluid; the flow will sometimes be steady for a while, and then a sudden reversal will take place in the direction of the current, before it has reached an extremity. The gastric cavity is traversed by the stream, as well as all portions of the hydrasoma. In some species the cause of the flow has revealed itself under the microscope. The cavities through which the current moves are seen to be clothed with cilia—tiny lashes whose rhythmic motion forever propels the fluid; this ciliary action is no doubt greatly aided by the contractility of the walls. In many species the cilia, if there be any, are too minute for detection; but it is a fair provisional inference that where