PRINCIPLES Of PHYSIOLOGY \\l 



sense organs and their individual sensitive cells are connected to particu- 

 lar parts of the brain. 



Coordination and Integration. The activities of the several parts 

 of a many-celled organism must be coordinated if that organism is to 

 survive, and the greater the degree of complexity, the greater the 

 specialization of the parts, the greater is the need for precise integration 

 of their separate functions. Coordination of activity is achieved by two 

 major systems, nervous and endocrine. The nerves and sense organs 

 provide for rapid and precise adaptation to environmental factors. The 

 endocrine system, the glands of internal secretion which secrete sub- 

 stances into the blood stream (or its equivalent in lower animals), pro- 

 vides for less rapid, but longer lasting adaptations such as general body 

 growth, differentiation, development of sex organs and mating behavior, 

 responses to stress, control of tissue metabolism and regulation of pig- 

 mentation. The nervous mechanisms such as reflexes by which coordina- 

 tion and integration are achieved will be discussed in Chapter 29. 



The substances secreted by endocrine glands, called hormones, can- 

 not be defined as belonging to any particular class of chemicals; some 

 are proteins, some are amino acids and some are steroids. They are 

 distinguished as a group as being substances secreted by cells in one part 

 of the body which are carried by the blood stream to some other part 

 where they affect cell activities in a definite and characteristic fashion. 

 Acetylcholine and sympathin fit this definition of a hormone and are 

 sometimes referred to as neurohormones to emphasize this. AV^hether a 

 hormone will affect a specific tissue, and the nature of the effect pro- 

 duced, is a function of the tissue; each tissue will respond only to certain 

 hormones. In general, hormones produced in one animal will affect the 

 cells of other animals in related species, orders and even, in some cases, 

 classes. The endocrine glands of the vertebrates will be discussed in 

 Chapter 30. 



The processes under endocrine control in invertebrates include 

 molting, pujKition and metamorphosis in arthropods, pigmentation in 

 molluscs antl arthropods, and growth and differentiation of secondary 

 sex characteristics in annelids and arthropods. The development of 

 insects, by a series of molts and metamorphoses, is controlled by two 

 hormones, the "growth and differentiation" hormone (GDH) and "ju- 

 venile hormone" (JH). GDH is secreted by certain cells in the dorsal 

 mid-region of the insect brain and induces molting accompanied by 

 metamorphosis; juvenile hormone is secreted by the corpus allatum, a 

 single median gland in the posterior head region, and inhibits meta- 

 morphosis. Transplantation of corpora allata into developing insects 

 prevents metamorphosis for several successive molts, so that giant adults 

 eventually result. In moths such as the silkworm, Platysamia cecropia, 

 the situation is even more complicated: a hormone secreted by the brain 

 stimulates the prothoracic glands to secrete a second hormone which 

 ends the pupal period and brings about metamorphosis by stimulating 

 the cytochrome system of enzymes. 



The molting of crabs and other crustaceans is a complex process 

 involving many biochemical processes which must occur in proper 



