96 GENERAL CONCEPTS 



shells, or secretions of mucus or cutin. Hair, feathers and certain scales, 

 such as those of reptiles, are composed of very insoluble proteins called 

 keratins derived from dead cells in the skin. The skin has a number of 

 functions: it protects the underlying protoplasm against mechanical 

 and chemical injuries; it prevents the entrance of disease organisms; it 

 prevents excessive loss of water from land animals and excessive uptake 

 of water by fresh-water animals; and it protects underlying cells against 

 the harmful effects of the ultraviolet rays in sunlight. 



The skin is an effective radiator by which the body can eliminate 

 the heat which is constantly produced in cellular metabolism. One of 

 the factors controlling the rate of heat loss in higher vertebrates is the 

 size of the blood vessels in the skin. To conserve heat in a cold en- 

 vironment, the blood vessels are constricted to decrease the rate of 

 blood flow. The reverse occurs in a warm environment and the rate of 

 heat loss can be increased by the evaporation of water, i.e., sweat, from 

 the surface of the skin. 



A great many animals have a firm framework or skeleton which 

 protects and supports the body and provides for the attachment of 

 muscles. Some animals manage to survive without a skeleton but these 

 are mostly aquatic forms. The slug and earthworm are among the few 

 exceptions to the rule that terrestrial animals require a skeleton. To 

 raise part of the body off the ground, some stiff, hard framework is re- 

 quired to support the soft tissues against the pull of gravity. The ap- 

 pendages of arthropods and vertebrates have a hard, but jointed and 

 bendable, skeletal framework which serves as levers for locomotion. The 

 skeleton also covers and protects such delicate organs as the brain, 

 spinal cord and lungs. The marrow cavities of vertebrate bones con- 

 tain tissues which produce red blood cells and certain of the white blood 

 cells. 



An animal's skeleton may be an exoskeleton, located on the outside 

 of the body, or an endoskeleton, located within the body. The hard 

 shells of lobsters, crabs and insects, and the calcareous shells of oysters 

 and clams, are examples of exoskeletons. An exoskeleton provides ex- 

 cellent protection for the body, and muscles can be attached to its inner 

 surface so as to move one part with respect to another. However, the 

 presence of an exoskeleton usually interferes with growth. The arthro- 

 pods have solved this problem by periodically shedding the shell. To 

 do this the shell is first softened, that is, some of the calcium salts 

 deposited in it are dissolved, the shell is split and the animal crawls out 

 of the old shell. It then undergoes a period of rapid giowth before the 

 new shell, which formed under the old one, becomes hard by the depo- 

 sition of calcium salts. During this molting process the arthropod lacks 

 protection and is weak and barely able to move. Hard, calcareous exo- 

 skeletons are present in most molluscs and arthropods, and in corals, 

 bryozoa and a variety of lesser invertebrates. A clam or oyster secretes 

 additional shell at the margin as it grows; the shell gets both larger 

 and thicker as the animal grows. Many marine worms secrete calcareous 

 tubes in which they live. Though these shells are not directly a part 



