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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 front, where the crab 



