MARINE BIOLOGICAL STATION AT POUT ERIN. 93 



straightening their muscular and many- jointed, but 

 fragile, arms. This group is usually represented in our 

 tanks by Ophiothrix fragilis (fig. VIIL, 3), which lives 

 under stones at extreme low-water mark as well as in deep 

 water, where it is sometimes very abundant. A small 

 species called Amphiura elegans, found in rock pools 

 amongst Corallines and under stones, is a strikingly 

 beautiful object when viewed under a microscope. 



Echinlds, Sea-urchins : Almost every visitor to Port 

 Erin is familiar with the common sea-urchin, Echinus 

 esculentus (fig. YIIL, 5), either living on the rocks and 

 the breakwater at low tide, or as dried and spineless shells 

 exposed for sale outside the cottages of the fishermen. 

 The shell is composed of a large number of calcareous 

 plates, firmly joined together by their edges and arranged 

 in ten double rows which run from pole to pole of the 

 shell. All these plates bear small projecting knobs, to 

 which the spines are attached, and those plates which 

 form the five smaller of the double rows are perforated by 

 pairs of minute holes, through which the watery fluid 

 contained in a system of tubes inside the shell passes out 

 into the tube-feet. These latter are the organs of locomo- 

 tion, and may generally be seen in action on the living 

 urchins in the aquarium tanks. The mouth is in the 

 centre of the larger, flatter pole of the shell, and is armed 

 with five powerful jaws, which together form a rounded 

 pyramid, commonly called Aristotle's lantern. Each jaw 

 carries a long pointed white tooth, which projects from 

 the mouth. The smaller, dull-green urchin which is 

 generally on view in our tanks is Echinus miliaris. It is 

 found in rock pools and under stones at low tide. The 

 fragile " heart " urchin, Echinocardium cordatum, burrows 

 a few inches below the surface of the sand exposed only 

 at low spring tides in the centre of the beach, and, as it 

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