14 Elementary Zoology. 



by this particular pigment. Green birds and green lizards, for example, do 

 not in any case owe their colour to chlorophyll. 



In order to prove conclusively that a given green pigment is or is not 

 chlorophyll, it should be submitted to three tests— chemical, physiological, 

 and morphological. 



Chlorophyll is, as a rule, associated with certain structures, the 

 chlorophyll corpuscles, which are nucleus, like masses of protoplasm 

 tinged with the chlorophyll. This is not the invariable case ; but it is 

 safe to regard a green pigment contained in special corpuscles as being 

 chlorophyll, though the converse cannot be asserted. This may be termed 

 a morphological test. 



Chlorophyll has certain definite chemical reactions and characters. In 

 the first place it shows a definite absorption spectrum with characteristic 

 dark bands. It is soluble in alcohol, and the solution is fluorescent. By 

 transmitted light it is green, by reflected light reddish. There are, of course, 

 a variety of other chemical methods of deciding whether the pigment is 

 chlorophyll. 



Finally, there is the physiological method. Protoplasm can, in the 

 presence of chlorophyll, split up the carbonic acid of the air into oxygen 

 and carbon, combining the carbon with the water in the protoplasm to form 

 — usually starch, but sometimes some other substance, e.g. oil, composed 

 of the elements carbon, oxygen, and hydrogen. 



Tried by the first two of these tests, the green colour of hydra is 

 chlorophyll. 



Experiments, however, appear to show that it is of no great physiological 

 use to its possessor. 



Opinions differ as to whether the chlorophyll is really a product of the 

 cells of the hydra (in which case there is a most interesting point of like- 

 ness between animal and vegetable protoplasm), or whether the so-called 

 chlorophyll corpuscles of hydra are not to be looked upon as small 

 unicellular plants. If so, this illustrates what is termed symbiosis ; as the 

 small plants derive advantage (shelter, etc.) from their association with the 

 host, while (though, as already said, this is not so clear) they confer 

 advantages in the shape of starchy or other matters formed by them upon 

 their host. 



When a transverse or longitudinal section is made through 

 the hydra, the body is seen to consist of two layers of cells 

 surrounding a central cavity, which latter communicates with 

 the exterior through the mouth aperture before referred to. 

 When a hydra is teased up in water with fine needles, these 

 cells are dissociated from each other, and float freely about. 

 Each may then be seen to consist of a piece of protoplasm, with 

 a more or less centrally placed nucleus. The boundaries of the 

 cells may also be recognized in the sections. The whole body 

 of the hydra is thus built up of a number of structural units or 

 cells, each one of which is the equivalent of a single amoeba or 

 vorticella. The hydra, and all the animals lying above it in the 

 series, is " multicellular ; " the amoeba and the vorticella are 



