122 BIOLOGY. [BOOK ir. 



forming. The same general remark may be applied to a number 

 of infusoria, the amoebae, and the rhizopods. 



At a later period we shall have to describe the principal 

 modes of anatomical and physiological differentiation, by means 

 of which the complex animal organisms prepare the way for 

 interior nutrition, and we shall see them gradually working, 

 becoming more and more complicated in proportion as we rise 

 in the animal series. For the present, we have only to consider 

 the general nutritive phenomena ; the principal exchanges of 

 matter, which are accomplished between a complex animal and 

 the exterior world. 



In such an organism, there is no direct interchange between 

 the anatomical elements and the ambient medium ; intermediary 

 liquids, which may be compared to the elaborated sap of superior 

 plants, have the office of receiving the different kinds of nutriment 

 fully prepared for assimilation, of conveying them of presenting 

 them, as it were, to the anatomical elements ; the latter, by 

 virtue of their innate constitution, of their affinities, draw from 

 these interior and living mediums the substances which suit 

 them, and reject those of their molecules which are altered by 

 the action of life. 



We have already shown what is the composition of those 

 living mediums called plasmas and blastemas. We know that 

 therein are found three orders of immediate principles, mineral 

 substances, ternary matters, amyloidal, saccharine, fatty, finally 

 and especially substances proteic or albuminoiidal. Naturally, 

 the diverse chemical compounds of living liquids are so much 

 the more numerous, the more varied, in proportion to the 

 complexity and differentiation of the animal, since in every 

 organism each kind of anatomical element has its special 

 alimentary preferences. Also, whilst amongst the superior 

 plants we find some products of dis-assimilation little varied, 

 as for instance asparagin, and perhaps some vegetal acids; we 

 see among the superior animals the proteic substances alone 

 furnishing, when the anatomical elements reject them after 



