xx GENERAL INTRODUCTION. 



be devoid of chlorophyl bodies, but its other characters remain unchanged. 

 If it has a motile initial stage the course of development proves its plant- 

 like nature. In those rare instances, i.e. in Insectivorous plants where 

 solid food is digested and the products of digestion utilised, the process 

 of digestion is carried on externally to the organism, and absorption takes 

 place by the outer surface. 



What is true of multicellular animals and plants is true, within limi- 

 tations, of unicellular. There are of course in a unicellular animal no 

 specialised systems of organs such as the digestive, for example, but power 

 of locomotion remains, and the natural irritability, automatism and con- 

 tractility of the protoplasm are very strongly developed. Solid organic 

 food is ingulfed within the protoplasm and is broken down, giving rise 

 to fat, albumen, glycogen or other starchy bodies as in higher animals ; 

 it also leaves generally a faecal residue, and there is reason to think 

 that complex and sometimes crystalline excretory products are formed. 

 But the organism may in some cases utilise organic food in solution, 

 in other words it is saprophytic, e.g. some Flagellata, and probably the My- 

 cetozoa ; in other cases, e. g. the Flagellate Englena, owing to the presence 

 of chlorophyl bodies, nutrition becomes holophytic or completely plant- 

 like. In these instances recourse can be had only to considerations of 

 structure, life-history, comparison with other forms, or the behaviour of 

 the doubtful organism under altered conditions of life. Good examples 

 of these considerations may be drawn from Flagellata and My cetozoa. The 

 position however of some few forms, e. g. the Volvocina, remains a matter 

 of doubt, and they are claimed by botanists and zoologists alike. Their 

 nutrition is holophytic, and their structure is paralleled in undoubted 

 vegetable organisms 1 . 



However complex in structure a multicellular animal or plant may 

 be, it can be traced without exception to an origin from a single cell. 

 Many animals, the whole group known as Protozoa, and many plants 

 never attain a higher degree of morphological complexity than a single 

 cell. But in some Protozoa,, at any rate, that cell possesses highly de- 

 veloped vital energies and a corresponding specialisation of parts. In its 

 simplest aspect a cell may be defined as a mass of protoplasm (cytoplasm) 

 containing one or more nuclei. It has been shown that non-nucleated 

 masses of protoplasm, derived from nucleated, are in the Protozoa capable 

 of growth in size, but they have no power of reproduction. On the other 

 hand, there are a few Protozoa (certain Proteomyxa] which appear to 



1 See MaupaS; C. R. 88, 1879, p. 1274. 



