THEORY OF THE DEVELOPMENT OF ORGANISMS 111 



the stem and branches. The different mode of 

 nutrition of animals results in a totally different 

 structural plan. Animal cells absorb material that 

 is already organised, and that they may do so their 

 cells are either quite naked, so affording an easy 

 passage for solid particles, or they are clothed only 

 by a thin membrane, through which solutions of 

 slightly diffusible, organic colloids may pass. 

 Therefore, unlike plants, multiceliular animals 

 display a compact structure with internal organs 

 adapted to the different conditions which result 

 from the method of nutrition peculiar to animals. 

 A unicellular animal takes organic particles bodily 

 into its protoplasm, and forming around them 

 temporary cavities known as food vacuoles, treats 

 them chemically. The multiceliular animal has 

 become shaped so as to enclose a space within its 

 body into which solid organic food -particles are 

 carried and digested, thereatter, in a state of solution, 

 to be shared by the single cells lining the cavity. In 

 this way the animal body does not require so close 

 a relation with the medium surrounding it ; its food, 

 the first requirement of an organism, is distributed 

 to it from inside outwards. In its further complica- 

 tion the animal organisation proceeds along the same 

 lines. The system of internal hollows becomes 

 more complicated by the specialisation of secreting 

 surfaces, and by the formation of an alimentary 

 canal, and of a body cavity separate from the 

 alimentary canal. 



In plants, it is the external surface that is in- 

 creased as much as possible* In animals, in obedi- 



