. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. ila septentrionalis. * In the tentacula of some of the physograda, also, a similar extensibility exists. The lower sur- face of physalus, for instance, which itself seldom exceeds six inches in length, is provided with ten- tacula sixteen and even eighteen feet long. partly by the movements of the tentacules which hang down from the inferior surface; but chiefly, perhaps, by the action of the wind on the raised crest, with which most of these animals are provided. Immediately around the mouth are placed numerous small tu
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. ila septentrionalis. * In the tentacula of some of the physograda, also, a similar extensibility exists. The lower sur- face of physalus, for instance, which itself seldom exceeds six inches in length, is provided with ten- tacula sixteen and even eighteen feet long. partly by the movements of the tentacules which hang down from the inferior surface; but chiefly, perhaps, by the action of the wind on the raised crest, with which most of these animals are provided. Immediately around the mouth are placed numerous small tubular suckers, similar to the feet of many echinoder- mata. Exterior to these there are longer tenta- cula, for the most part in a single row, and simple; sometimes branched. Neither of these two kinds of organs is very extensile. The disc from which the tenlacules hang, and the crest, are supported internally by a calcareous plate, which is the only organ of the kind in the whole class of acalephse. It somewhat re- sembles in structure the calcareous axis of retepora, being cellular and porous. Its nu- merous cells are filled with air, which renders the whole animal so buoyant that it floats on the surface of the water, and is wafted along by the winds. In velella (Fig. 11.) there are two plates, one placed horizontally, the other perpen- dicularly upon the upper surface of the former. They are marked with lines of growth, enlarg- ing from within outwards, like the extravascular shells of the mollusca. The perpendicular plate in velella supports the crest, which stands upright, and exposes a large surface to the wind. Rataria (Fig. 12.) has its crest provided with strong muscular bands run- ning perpendicularly. It lies on the surface of the water, with the crest stretch- ed out, so that its whole side touches the water. When it is alarmed, the crest is suddenly contracted, and the centre of gravity is so al- . tered in consequence, that ^tana cordata. the position of the bod
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