CAPTURE OF ANIMAL FOOD 79 



polyps, sea-squirts and bivalves, whose irrigation 

 works are ceaselessly straining the water, filtering off 

 and digesting the drift life. Between the two con- 

 stituent kinds of filtrates, plant and animal, they do 

 not distinguish, but swallow and digest them both. 

 A plant, however, is more difficult to digest, as around 

 each element of its tissues there is a tough covering 

 such as animals only possess on their outer surface. 

 Hence an animal once captured and torn is quickly 

 digested, whereas a marine or aquatic plant is only 

 dissolved with great difficulty. We are not surprised, 

 therefore, to find that plants may live for a consider- 

 able time in the body of an animal, and, on the other 

 hand, that animal food, being juicy and stimulating, 

 would force the growth of those fixed creatures which 

 attempted the diet. The hydroids, jelly-fish, and 

 anemones have made both these discoveries, and the 

 larger they grow the more confirmed becomes their 

 carnivorous habit. Little by little the power of 

 holding and then numbing moving prey was deve- 

 loped. Stickiness was a necessity to prevent the 

 tissues becoming waterlogged and the cilia from 

 flagging, and to this slimy covering these resourceful 

 creatures added the power of poisoning their captives 

 by the aid of nettle-cells, with which jelly-fish and 

 anemones sharply annoy us when bathing or wading. 

 To digest this animal food was no new or great dif- 

 ficulty, for the lower animals feed upon their own 

 tissues when other food is lacking, as it often is in 

 winter, and a jelly-fish the size of a saucer becomes, 



