80 General Part. 



Appendix. 



Resemblances and Diflferences between Plants and 



Animals. 



Now that tlie most important features in the structure of animals in 

 genei-al have been stx\died, a consideration of the relations of the Animal 

 Kingdom to the other great division of the organic world, the Vegetable 

 Kingdom, may not be out of place. 



Both plants and animals are composed of cells; in the simplest forms of 

 a single cell only, but usually of a great number. The cells consist, at least when 

 young, of protoplasm, and generally (perhaps always) in plants as well as 

 animals, contain a nucleus. The protoplasm of plants displays the same 

 essential characters as that of animals (see pp. 1 — 1) ; it exhibits the power of 

 movement ; it possesses irritability ; it feeds, by taking up. materials from the 

 envii'onment ; it absorbs oxygen and gives ofB carbon-dioxide. The cells grow, 

 and miiltiply by fission. Sexual reproduction, i.e., the formation of a new 

 organism from a single cell after its union with one from another individual 

 of the same species, occm-s in most plants as well as in most animals. 



In considering the differences between the two kingdoms, the lowest 

 unicellular plants and animals, the Protophyta and the Protozoa will, at first, be 

 disregarded, whilst the Metazoa and the multicellular plants ai-e contrasted. 



In general construction there are important differences. Animals, with 

 very few exceptions, possess an alimentary canal, a cavity into which food 

 is taken, and in which it is digested and absorbed ; it is specially characteristic 

 of this system that it appears at a very early stage of development. Anything 

 homologous, or even only analogous with, the digestive canal is entirely wanting 

 in plants, which take in their food through the sui-face in a liquid or gaseous 

 form. This is not, however, an absolute distinction, for an alimentary canal 

 is absent from some animals {e.g., the Tape- worms). Further, a muscular 

 and anervous system are wanting in all plants, whilst these organs apparently 

 occur in all Metazoa: sense organs, which iu their simplest form, as sense- 

 cells, are probably represented in all Metazoa, are never present in plants : a 

 vascular system and special excretory organs are also peculiar to the 

 Animal Kingdom, although they do not occur in every member of it. On the 

 whole, it may be said that plants exhibit ia internal organisation, only a very 

 slight indication of that specialisation of organs which is, comparatively, so 

 pronounced in animals. The so-called organs of plants are only special regions 

 and appendages of the body. 



Quite as significant is the contrast in the tissues composing the body. 

 In animals, the cells, originally undifferentiated, develop ia very different 

 ways. Some remain in their primitive condition, others secrete an intercellular 

 substance, varying in stru.cture and chemical composition ; in others, the proto- 

 plasm becomes modified into a pectdiar contractile substance. In plants, 

 the cells are almost always sui-rounded with cell- walls, consisting of cellulose, 

 and the internal differentiation of the body, as far as its tissues ai-e concerned, 

 depends, principally, upon a variation in the form of the cell or upon the thick- 

 ness, toughness, etc., of the cellulose wall, and less upon modifications of the 

 protoplasm, which always remains protoplasm, unless, as in many adult cells, it 

 has disappeared entirely. 



