INTRODUCTION 



Animal cells rarely show so definite a cell-wall, and this, when 

 present, is not of cellulose, but of a nitrogenous compound more 

 nearly resembling the cytoplasm. Very often in animal tissues the 

 boundary of the individual cell is not clearly marked, and the result 

 is a " syncytium," or multinucleate protoplasmic mass, rarely found 

 in plants. 



Movement in Plants. The development of a firm membrane about 

 the cell interferes, of course, with its motility, and we thus find 

 plants, as a rule, much less motile than animals, this being especially 

 true of the larger multicellular forms. 



The lower plants, especially many unicellular forms, are often 

 actively motile, the movements being due to the vibration of deli- 

 cate protoplasmic threads (cilia), which are either prolongations of 

 the naked cell-body, or pass through openings in the cell-wall. By 

 means of the cilia, the plant swims freely in the water like an Infu- 

 sorian. The possession of cilia in the ordinary vegetative condition 

 of the plant is confined to a comparatively small number of the 

 lower forms ; but these often show at times a passive stage, e.g. the 

 so-called "Palmella" stage of certain Volvocaceae the " Zoogloea " 

 stage of many Bacteria. This latter condition becomes the rule in 

 all the higher plants, and only the reproductive cells show a rever- 

 sion to the free-swimming, ciliated type. With the assumption of 

 the non-motile vegetative conditions, the stationary character of the 

 typical plant-organism is established. 



Motility in Animals. The case is different with animals. In these 

 the active cells remain permanently naked, or at any rate destitute 

 of a rigid membrane. In consequence, the cells are capable of much 

 greater change of form and size than is ever the case with plants. 

 The power of spontaneous locomotion in plants becomes less marked 

 as differentiation proceeds, and in the highest forms is entirely lost. 

 In animals the reverse is true, and the most highly specialized forms 

 show most perfect motility. We rightly, then, consider locomotion 

 as a distinctly animal attribute, although not confined exclusively to 

 the animal kingdom. 



The power of locomotion is no doubt associated with the question 

 of food. Plants being able to use the inorganic compounds derived 

 from the atmosphere and earth, which are renewed from time to 

 time, and above all the power of green plants to utilize the energy of 

 the sun's rays, make it unnecessary for them to move away from the 

 spot where they grow ; and except for the establishment of new indi- 

 viduals, they do not develop means of locomotion. A few animals, 

 like the Corals and many Mollusks, where the currents of water bear 

 them renewed supplies of food, behave in this respect much like 

 plants ; but most animals must be able to range over a large area in 

 order to obtain the food necessary to support life. 



