164 ESSENTIALS OF ZOOLOGY 



them, lies parallel to the spinal cord, and constitutes the autonomic 

 system. The peripheral system includes ten to twelve pairs of cranial 

 nerves from the brain, and ten to thirty-one pairs of spinal nerves in 

 different forms of vertebrates. Each spinal nerve has two roots 

 where it joins the spinal cord. 



A high development of sense organs for the senses of sight, hear- 

 ing, smell, taste, and touch is characteristic of vertebrates. The organs 

 are receptors and they are stimulated by changes in external environ- 

 mental conditions, such as light, sound waves, chemical changes, and 

 contact. The eye, which is the organ of sight, is a highly developed 

 organ. It is constructed on the plan of a camera with the eyeball 

 forming the light-tight box. 



The ear structures provide most classes of vertebrates with facilities 

 for two functions : hearing and equilibrium. This organ consists of 

 an external ear, which serves in catching and directing sound waves 

 within, a middle ear or tympanum, containing ossicles, and the inner 

 ear, which contains the sensory cochlea with its organ of Corti for 

 hearing, and the semicircular canals, which are concerned with equi- 

 librium rather than hearing. The latter are common to all verte- 

 brates while the cochlea is limited to Amphibia and higher classes. 



The sense of smell is centralized in the epithelial lining of the nasal 

 chamber. Special olfactory cells are stimulated by particles of mate- 

 rial from the air dissolving on this membrane and making contact 

 with the sensory cells. The sense of taste is similar except that it 

 is located in sensory cells in taste buds on the tongue, epiglottis, and 

 lips (and barbels of some vertebrates). The particles come in by way 

 of food and drink and as the material dissolves, it reaches the taste 

 cells. 



Most of the tactile and pressure sense organs are located just 

 beneath the skin over different parts of the body. A few of the 

 pressure sense organs are found in certain of the internal structures 

 of the body. The lateral line system in fishes is sensory to vibrations 

 carried in the water and is quite important to aquatic animals of 

 this type. 



The vertebrate reproductive system shows a fairly high degree 

 of development. The sexes are ahiiost universally separate, with 

 the exception of some cyclostomes. The distinct gonads develop to 

 produce special germ cells. The male gonads are testes, and they 

 produce spermatozoa which are carried from the gonads by the vasa 



