72 VITAL ANIMAL LIFE PROCESSES 



Reproductive System. This system includes special organs which 

 have the specific function of propagation of the species. 



Nervous System. This system coordinates the various parts of 

 the body. In the simpler animals it may consist of only a network 

 of nerves, but as animals become more complex the nervous system 

 becomes much more extensive. There may be a variety of sense organs 

 which pick up stimulations of different kinds, a nerve center known 

 as a brain, a nerve cord, and nerves running to all parts of the body. 



Muscular System. In the simplest of animals all of the cells have 

 the power of contraction and all share in producing body movements. 

 As cells become specialized, however, they tend to lose this power 

 and special cells are set aside to take care of movements of the body. 



Circulatory System. This system becomes necessary as animals 

 become so large that materials cannot be distributed over the body 

 by simple diffusion. A circulatory system consists of a fluid, usually 

 designated as blood, which is kept moving in a circuit around the body 

 by some pumping force, such as a heart. 



Skeletal System. This system supports the soft parts of the body 

 and protects them from injury. Muscles are attached to the skeleton 

 as an anchorage. The skeletal system may be divided into two major 

 types. The exoskeleton is found on the outside of the body and is 

 best illustrated by the hard outer covering of the insects. The endo- 

 skeleton is found inside the body and is illustrated by the human 

 skeleton. 



Integumentary System. This system includes the skin and its 

 numerous appendages, such as hair, nails, spurs, horns, hoofs, and 

 glands. The primary function of this system is the protection against 

 infection, water and heat loss, and abrasion. Many aquatic animals 

 also use their skin for respiration. 



Endocrine System. This system is comprised of a series of glands 

 in various parts of the body and they secrete products called hormones 

 which regulate many important body activities. For instance, there is 

 a thyroid gland located in the neck of man which secretes a hormone 

 that regulates the rate of metabolism in cells all over the body. Hor- 

 mones are spread over the body by means of the blood circulation. 



As we survey the animal kingdom it will be interesting to note the 

 development of these various organ systems. 



REVIEW QUESTIONS 



1. Name the four vital life processes necessary for all animals and tell why 

 each is necessary. 



2. Distinguish between intracellular digestion and extracellular digestion. 



