SEA-ANEMONES AND THEIR ALLIES. 71 



a resemblance to sand. The column is long, often very long, 

 and cylindrical, and in its upper two-thirds is covered by 

 distinct suckers, to which fragments of stone and shell are 

 often attached. On the other hand, in a sandy pool it is 

 not uncommon to find specimens which, instead of being 

 covered with isolated fragments of gravel, have a complete 

 investment made of fine sand glued together by mucus. 

 This can be peeled off, and leaves the smooth column below. 

 The cave-dweller does not always live in sandy pools, but, 

 as the name indicates, is often found in rock crevices. 

 There the colours are brighter, the prevailing tint being 

 greenish brown or grey-violet. They are also often abundant 

 in the beds of young mussels which sometimes cover the 

 flat surfaces of rocks. The young mussels are, as it were, 

 embedded in a thick layer of silt, which intervenes between 

 them and the rock surface. The anemones are attached to 

 the rock like the mussels, and protrude their starry crowns 

 through the layer of silt, while the shells of the mussels 

 make a firm wall around them. If you look at such a young 

 mussel-bed from a distance of a few feet, you will notice 

 that the uniformity is interrupted by numerous rounded 

 spaces, in which the silt and sand show out in contrast to 

 the dark shells of the surrounding mussels. A close 

 examination will show you that each gap is occupied by a 

 flourishing Sagartia troglodytes. The sight is an interesting 

 one, and suggests many problems. What does the anemone 

 gain from the association? Is it a true case of an "animal 

 association," or is it merely a chance that the same environ- 

 ment should suit both? Does the anemone obtain any of 

 those benefits somewhat vaguely summed up in the word 

 protection, or is it that it shares the food of the mussels'? 

 These are only a few of the questions one would like 

 answered. 



The cave-dweller lives so well in confinement, and its 

 markings so well repay study, that a few should be taken 

 home for the purpose. In one habit it differs remarkably 

 from the two preceding anemones. They will rarely expand 

 freely unless the base is firmly fixed, and as every aquarium 

 keeper knows, they will never thrive unless they can be 

 persuaded to attach themselves almost at once. The cave- 

 dweller, on the other hand, rarely completely retracts its 



