35 ZOOLOGY. 



cated sense-organs are removed from the surface and are much 

 modified, but the essential sensory portion is similar in all, 

 and they retain some suitable connection with the outside. It 

 is usually these accessory connecting structures which render 

 the sense organ so complicated. 



364. The Skin Senses. Scattered over the body of many 

 forms of animals are single cells, or groups of cells, or free 

 nerve endings, which are for the reception of contact and tem- 

 perature stimuli. These are not equally numerous or well 

 developed in all parts of the body. They are often especially 

 developed in connection with hairs. In the lower aquatic 

 vertebrates, especially the fishes, groups of such sensory cells 

 occur in pits or longitudinal grooves along the sides. These 

 are called the organs of the lateral line. Their exact function 

 is still in some doubt. They may possibly assist in the de- 

 termination of the chemical condition of the water or in 

 determining the equilibrium of the animal in the water. 



365. The Chemical Senses Taste and Smell. The 

 chemical senses involve close contact and a chemical union 

 between the substance to be perceived and the organ itself. 

 For that reason the substance must be capable of solution in 

 the fluids that moisten the surfaces. In vertebrates these 

 organs are located at the anterior end of the body and usually 

 within special pits or cavities. The taste organs are in the 

 mouth, especially on the tongue and soft palate. In some ani- 

 mals the sense is poorly developed. The end organs of the 

 sense of smell are located in pits, anterior or dorsal to the 

 mouth, lined with folds of the mucous epithelium. In most 

 fishes these pits are not connected with the pharynx, but in all 

 air-breathing forms there is such connection, and the nostrils 

 constitute the normal inlet for air to enter the lungs. The 

 sense of smell is much more developed in the air-breathing 

 vertebrates, if indeed it can be said to exist at all in the aquatic 

 animals. 



366. The Ear. The vertebrates have a single pair of ears, 

 and these are located at the side of the head behind the 



