PHYSIOLOGY OF THE NERVOUS SYSTEM. 



767 



cells connected together to receive and convey impression (Pig. 321). 

 It consists of a ring around the mouth composed of five ganglia of equal 

 size with radiating nerves. From this circle delicate fibres may be traced 

 into the different rays. In lower zoophytes all traces of the nervous 

 s^ystem is wanting, and in them the functions of nutrition are accom- 

 plished through the operations of undifferentiated protoplasm. 



In the mollusks the nervous system has reached a sounewbat higher 

 form of development, and two or more ganglia are found located around 

 the gullet and communicating by nerve-fibres with other ganglia in dif- 

 ferent regions of the body, sending off nerves to the different organs. 

 Usually in the mollusks there is a cir- 

 cular ganglion located in the cephalic 

 side of the animal and two abdominal 

 ganglia placed below the oesophagus 



Pis. 321.— Nervous System of the Star- 

 Fish. (Cams.) 



Fig. 322.— Nervous System of a Gas- 

 teropod Mollusk. (Perrier.) 



c, cerebroid ganglia ; p, pedal ganglia ; o, otocysts ; 

 v,vl,vtt, ganglia of second (esophageal collar; I, ten- 

 tacles ; y, ej-es ; X, excrement. 



and united to the cephalic ganglion and the oesophageal ring (Fig. 322). 

 These ganglia are frequently connected with others whose locations will 

 vary in different species. 



In the articulata, represented by the insects, annelida, and crusta- 

 ceans, the nervous sj'stem has become symmetrical (Fig. 323). 



The ganglia which compose the nervous system may be arranged 

 in pairs on each side of the median line of the body, each pair corre- 

 sponding to a segment of the body, extending throughout its entire 

 length and united to each other so as to form a longitudinal chain 

 of ganglia, which are connected further by transverse commissures. 

 Sometimes the ganglia consist of a single median row. Usually one of 

 the ganglia is more voluminous than the others, and being, as a rule, 

 located in the anterior extremity of the animal, might be compared to 



