THE GENERAL ANIMAL FUNCTIONS. 67 



into a resistant tissue (intercellular substance in bone, etc.), 

 or may be secreted at the surface of the organism as a whole 

 (cuticula in insects and shell in mollusks). The hard parts 

 serve primarily for the support and protection of the softer 

 tissues. Incidentally they come to serve a very important use 

 as points of attachment for muscles. The skeleton may be 

 external (the integumentary skeleton, or exoskeleton) as in 

 crayfish, or internal, as the endoskeleton of vertebrates. In 

 many instances both kinds of skeletal structures may occur 

 simultaneously, yet it is usually true that if the exoskeleton 

 is well developed the endoskeleton will be poorly represented. 

 Each has important advantages and limitations. To allow 

 motion as the result of muscular action the skeleton, if rigid, 

 evidently must be in segments and jointed. 



96. Growth. There are no special organs of growth, yet 

 growth is one of the most immediate and important manifesta- 

 tions of the nutritive process. Growth is to be defined as in- 

 crease in volume or mass and may result from either or all of 

 three processes: viz. (i) absorption of water, (2) formation 

 of protoplasm and the multiplication of cells, and (3) forma- 

 tion of non-protoplasmic cell-products, either within or among 

 the cells. 



The rate and character of growth are modified by such 

 external conditions as temperature, light, quantity and quality 

 of the food supply, etc. Growth does not continue indefinitely. 

 Its continuance is determined by the relation of the anabolic 

 to the katabolic processes in the body. The time comes in the 

 life of every complex organism when the income no longer 

 equals the outgo, and growth must cease. Later still the wear 

 is not made good by the income, and death results. 



97. Reproduction and the Reproductive Organs. Since 

 individual organisms are limited both with regard to growth 

 and length of life, it is apparent that a given class of forms 

 cannot continue, unless some method of originating new in- 

 dividuals be found. This production of new individuals by 



