c 7 2 THE FOOD OF ANIMALS 



a tube closed at one end, by which attachment is effected, and 

 open at the other, the aperture being the mouth, around which 

 a number of slender tentacles, used for catching food, are arranged 

 in a radiating manner. The animal is indeed little more than a 

 living stomach. The body-wall consists of two layers, an outer 

 (ectoderm) and an inner (endoderm), with which latter we are 

 here concerned. For imbedded in this layer are a large number 

 of chlorophyll-bodies, which almost certainly enable the animal to 

 live partly upon simple inorganic substances, though the chief 

 food consists of small animals captured by means of the tentacles. 

 Here again it is considered by some that the green granules are 

 really minute plants, but this remains to be proved. Saville Kent 

 (in The Great Barrier Reef of Australia) expresses the opinion 

 that many kinds of coral-animals live entirely in a plant-like way, 

 an opinion founded on the absence of animal substances in the 

 digestive cavities of specimens examined by him. The matter 

 needs careful investigation, and it is quite possible that these 

 creatures may turn out to subsist in the duplex manner supposed 

 to be characteristic of the Green Hydra. The fact that coral- 

 animals are often of the most brilliant colours other than green 

 is no difficulty, for even in some green plants chlorophyll is dis- 

 guised by the presence of other pigments, of which a good 

 instance is that of the common brown sea- weeds. If a bit of 

 such a plant be immersed in spirit for a short time it will turn 

 green, as the brown colouring matter is quickly dissolved out; 

 and perhaps chlorophyll is not the only pigment which possesses 

 the remarkable properties above described. 



Passing over the case of the Freshwater Sponge (Spongilti)* 

 which is of a green colour owing to the presence of chlorophyll, 

 we come to certain animalcules (Protozoa) in which this pigment 

 is present to a greater or less extent. Some of these are un- 

 doubted animals, and Euglena, which has been described above 

 (p. 267), is a good example of such cases. It partly feeds by- 

 taking in solid particles through a minute mouth, and also con- 

 tains chlorophyll-bodies, which no doubt enable it to utilize the 

 simple substances upon which green plants entirely subsist. From 

 a case like this we can pass to others where it is difficult, if not 

 impossible, to say whether we are dealing with plants or animals. 

 A familiar instance is afforded by a microscopic form which often 

 abounds in puddles, water-butts, &c., imparting a greenish tinge 



