Ecological animal geography; an authorized, Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage ecologicalanimal00hess Year: 1937 182 MARINE ANIMALS on the decomposing remains of the seaweeds and of other animals. This material is ingested together with the admixed sand by many forms; just as earthworms take in their organic food with the earth in which they live, so do the lugworm Arenicola, the hemichordate Balanoglossus, the holothurian Synapta, and various sea urchins. Others secure food with less admixture of sand by creating
Ecological animal geography; an authorized, Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage ecologicalanimal00hess Year: 1937 182 MARINE ANIMALS on the decomposing remains of the seaweeds and of other animals. This material is ingested together with the admixed sand by many forms; just as earthworms take in their organic food with the earth in which they live, so do the lugworm Arenicola, the hemichordate Balanoglossus, the holothurian Synapta, and various sea urchins. Others secure food with less admixture of sand by creating a current of water which brings them the finely divided particles of debris floating in the sea water. Amphioxus and many mollusks feed in this manner. Other mollusks, such as Scrobicularia (Fig. 17), draw in the fine deposit of food material on the surface of the sand by means of their long intake siphon, with which they search the surface about them. Fig. 17.—Scrobicularia piperata, in the sand; the afferent siphon to the left, the efferent or anal siphon to the right. After Meyer and Mobius. Breathing is a matter of special importance to these sand-dwelling creatures. The lamellibranchs and Amphioxus receive their oxygen with the water current which carries in their food particles. The sea urchin, Echinocardium (Fig. 18), produces a current of water by movement of its spines, which reaches it through a chimney-like tube, kept open by means of specially developed tube feet. The starfish Astropecten bears a series of small spines along its arms, whose vibrating motion forces the water along its surface. The lugworm (Fig. 19) possesses a series of much branched gills on both sides of the middle portion of its body, through whose extensive surface the haemoglobin of the blood efficiently takes up the scanty oxygen supply of its surroundings. Singular breath- ing arrangements are developed in the box crabs, which burrow in the sand. In them the water intake is situated in fro
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