178 AN INTRODUCTION TO ZOOLOGY 



peripheral nervous system (Fig. 90, Q and 10). The supra-pharyn- 

 geal ganglia supply the prostomium with two large nerves which 

 give off many branches (8); they also send nerves into somites 

 II and III. One nerve extends out from each circum-pharyngeal 

 connective. In each somite from IV to the posterior end of the 

 body, three pairs of nerves arise, two pairs from the ganglionic 

 mass and one pair from the sides of the nerve cord just behind 

 the septum which separates the somite from the one preceding. 



Each enlargement of the ventral nerve cord really consists of 

 two ganglia which are closely fused together. In transverse 

 section these fused ganglia are seen to be surrounded by an outer 

 thin layer of epithelium, the peritoneum, and an inner muscular 

 sheath containing blood vessels and connective tissue as well as 

 muscle fibers. Near the dorsal surface are three large areas, 

 each surrounded by a thick double sheath and containing a bundle 

 of nerve fibers. These are called neurochords or " giant fibers" 

 Large pear-shaped nerve cells are visible near the periphery in the 

 lateral and ventral parts of the ganglion. 



The nerves of the peripheral nervous system are either efferent 

 or afferent. Efferent nerve fibers are extensions from cells in the 

 ganglia of the central nervous system. They pass out to the 

 muscles or other organs, and, since impulses sent along them give 

 rise to movements, the cells of which they are a part are said to be 

 motor nerve cells. The afferent fibers originate from nerve cells 

 in the epidermis which are sensory in function, and extend into 

 the ventral nerve cord. 



The functions of nervous tissue are perception, conduction, and 

 stimulation (89). These are usually performed by nerve cells, 

 called neurons. The neuron theory " supposes that there is no 

 nerve fiber independent of nerve cell and that the cell with 

 all its prolongations is a unit or a neuron; that these units are 

 not united to one another anatomically, but act together physio- 

 logically by contact; that the entire nervous system consists of 

 superimposed neurons; . . ." (136, p. 633). 



The reflex carried out either consciously or unconsciously is 



