ONE-CELLED ANIMALS OR PROTOZOA 51 



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 organ'ic colloids may pass. 

 Therefore, unlike plants, multicellular 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 multicellular animal has 

 become shaped so as to enclose a space within 

 its body, into which solid organic food-particles 

 are carried and digested thereafter 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 complication the animal 

 organisation proceeds along the same lines. The 

 system of internal hollows becomes more com- 

 plicated 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 increased as much as possible. In animals, in 

 obedience to their different requirements, increase 

 takes place in the internal surface. The special- 

 isation of plants displays itself in organs exter- 

 nally visible in leaves, twigs, flowers, and ten- 

 drils. The specialisation of animals is concealed 



