16 



GE1STKEAL PABT. 



organ (kidney), mostly in a fluid form.' For the movement and 

 circulation of the fluid carrying the absorbed nutriment, there is a 

 pulsatory pump (heart) and a system of blood vessels, while respira- 

 tion is usually carried on in terrestrial animals by lungs, and in aquatic 

 animals by gills. Finally, animals possess internally placed generative 

 organs, and a nervous system, and sense organs for the production 

 of sensation. 



In plants, on the contrary, the vegetative organs have a much 

 simpler form. Roots serve to absorb fluid nutriment, while the 

 leaves act as respiratory and assimilating organs, taking in and giv- 

 ing out gas. The complicated systems of organs found in animals 

 are absent, and a more uniform parenchyma of cells and vessels, 

 in which the sap moves, composes the body of plants. The gener- 

 ative organs also are placed in external appendages, and there are 

 no nervous and sense organs. 



Nevertheless, the above mentioned differences are not universally 

 found, but rather hold only for the higher animals and plants, and 

 gradually disappear with the simplification of the organization. 



Even among vertebrates, and still more is it the case amongst 

 mollusca, and the lower segmented animals, the respiratory and 

 vascular organs are considerably simplified. The lungs or gills may 

 fail as special organs, and be replaced by the whole outer surface of 



the body. The blood vessels are 

 simplified, and sometimes they and 

 the heart are absent, the blood being 

 moved in more irregular streams in 

 the body cavity and- in the wall-less 

 spaces in the organs. Similarly, the 

 digestive organs are simplified ; 

 salivary glands and liver may no 

 longer be found as glandular appen- 

 dages of the alimentary canal. The 

 alimentary canal may become a 

 blind, branched, or simple sac 

 (Trematoda), or a central cavity, 

 the walls of which are in contact 

 with the body wall (Ceelenterata). 

 The mouth and alimentary canal 

 may also fail (Cestodes), nourish- 

 ment being taken in by osmosis 

 through the outer walls of the body as in plants. Finally, nerves 



Fie. 4. Branch of a Polyparium of 

 Corallhim rubrum (after Lacaze 

 Duthiers). P, Polyp. 



