I 



404 TEXTBOOK OF ZOOLOGY 



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 vibra- 

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

 of this type. 



Nervous Function — Reception and Conduction. — Irritability and 

 conductivity are fundamental functions of all protoplasm, whether 

 it be in the body of an Amoeba or a man. The responsiveness of 

 organisms to change of conditions both externally and internally 

 determines their behavior. Living protoplasm is not only excitable, 

 but it possesses the power to record or store up the effects of previ- 

 ous stimuli. In the final analysis, the perceptions and reactions of 

 man are but expressions of these primitive functions in a more 

 specialized organism. 



The protozoan organism has only neuromotor apparatus and de- 

 pends largely on the primitive properties of irritability and conduc- 

 tivity to guide its activities. In the simpler Metazoa, such as the co- 

 elenterates, there are scattered nerve cells connected with each other 

 by fibers to form a nerve net. The neuroepithelial or neurovmiscular 

 cells which make up this continuous net through the body are the 

 forerunners of the typical neurone and are called protoneurones by 

 Parker. A protoneurone transmits in every direction while a true 

 neurone transmits in only one. In the net system there is no central 

 exchange and no specific path of conduction. Every part of the 

 receptor surface of such an organism is in physiological continuity 

 with every other part of the body. 



Next comes the linear type of nervous system in the form of a 

 ladder. It is composed of an organization of neurones into a double 

 chain of ganglia, each cord lying lateral to the digestive tract with 

 transverse connectives and predominant ganglia at the anterior end. 

 Such a system was studied in planaria. In Annelida and Arthropoda 

 the nervous sj'stem is a modified ladder type in which the two longi- 

 tudinal cords of ganglia have fused along most of the midventral 

 line. Toward the anterior end, the cords separate at a paired gan- 



