I] PROTOPLASM 3 



them bodies in which certain changes take place, changes such as 

 growth, reproduction, movement, and others. 



A close study of animals reveals the fact that though the 



chemical constitution of no two is exactly alike, yet all contain 



certain allied highly complex substances of very 



Protoplasm. i i v i 



obscure chemical composition, known as proteids. 

 These substances occur in the form of a thick, viscous solution in 

 water; this is what is called by chemists a colloid solution or sol., 

 which on very slight provocation passes into a gelatinous solid, or 

 gel. This mixture of sol. and gel. is termed protoplasm (Gr. TT/OWTOS, 

 first ; TrXaor/xa, a thing moulded). 



Further, it has been found that, so long as any sign of life is 

 visible, this protoplasm is in a continual state of slow combustion, 

 absorbing oxygen from outside and decomposing with the liberation 

 of energy, and whilst some of the products of decomposition are 

 cast off, others apparently reconstitute the original substance by 

 combining with some of the materials of the food. The energy 

 liberated is the cause of the movements which constitute the visible 

 manifestation of life. 



An animal then is only the more or less constant form of a flow 

 of particles; it may be compared to a flame, which has a constant 

 form, although the particles which compose it vary from moment to 

 moment; unburned particles coming in at one end and the oxidised 

 products escaping at the other. 



/The deepest insight which can be obtained into the nature of 

 life viewed as a series of changes in the shape and 



Metabolism. . ^ 



position of bodies reveals to us this continual 

 chemical change as the ultimate cause of all manifestations of life. 

 It is known by the convenient name of metabolism (Gr. neTaftoXij, 

 change, changing). The ultimate object of Zoology is therefore 

 to discover the nature, cause, and conditions of the metabolism in 

 the case of every animal ; but the means of attaining this object 

 are still to seek, and for the most part the zoologist has to be con- 

 tent with describing and comparing with one another the outer and 

 visible effects of the metabolism in various cases. 



The proteids, which form the essential basis of protoplasm, 

 consist of carbon, nitrogen, hydrogen, oxygen, and sulphur; besides 

 these elements phosphorus, chlorine, potassium, sodium, magnesium, 

 calcium and iron are constantly found in the bodies of animals, and 

 some of them are doubtless chemically combined with the proteid. 



12 



