General Characters of A nimals. 3 



performs any special duties in the life of an animal 

 is called an organ. While there are thus varying 

 degrees of complexity among animals, yet the parts 

 of a simple animal have to perform as many essential 

 functions as those of a more complex animal, the in- 

 crease in complexity of an organism being correlated 

 not with an increase in the number of essential functions 

 but with the need for the more perfect fulfilment cf 

 existing duties. Increase in complexity thus results 

 from division of labour, and, with each increase, the 

 sphere of the functional activity of each part becomes 

 narrowed. For example, in jelly-fishes one set of 

 cavities acts as organs of digestion and of circulation, 

 while in higher animals these functions have separate 

 organs, and even subsidiary portions of these great 

 functions have for their accomplishment distinct parts. 



Functions. Three sets of functions are discharged 

 by organs in the body of an animal : namely, ist, 

 those of Relation ; 2nd, those of Nutrition ; 3rd, those 

 of Reproduction. 



The organs appropriated to the functions of Rela- 

 tion are those which connect the animal with its 

 environ 1 ng conditions, informing it about its surround- 

 ings, and enabling it to avoid disagreeable or to court 

 agreeable external influences. These organs are of two 

 kinds : (A) those of sensation, such as the skin, or organ 

 of touch, and the special sense organs (eye, ear, nose, 

 tongue), and (B) those of motion, which may be of 

 three kinds, (a) inconstant processes of protoplasm 

 called pseudopodia (fig. 8), (b) minute, constant, hair- 

 like processes having the power of waving to and 

 fro, called cilia^ or (c] contractile cells and fibres in 



