MULTICELLULAR ANIMALS 213 



141) is composed of a series of nerve cells that form a chain, the 

 axones of one being in contact with the dendrites of the next, and 

 so on. In the ccelenterate nerve net the protoplasm of the cells is 

 continuous, but in the more advanced types the communicating 

 axones and dendrites are in contact through a special structure 

 termed the synapse. The synapse is not thoroughly understood, but 

 it is known that impulses pass through in one direction only. 



Nervous Systems of Invertebrates. In general, the nervous 

 systems of animals higher in the scale than Coelenterata consist of 

 more or less centrally located groups of nerve cells whose fibres 

 ramify to all regions of the organism, either as receptors or associ- 

 ated with muscles or other effector organs. An aggregation of nerve 

 cells is called a ganglion. In animal forms that are not segmental 

 there is no regularity in the position of the ganglia in the different 

 types. For example, in the starfish many nerve cells are located 

 among the ectodermal cells; others form nerve trunks in each arm, 

 which are in communication with a central nerve ring encircling 

 the mouth (Fig. 142). In a clam the nervous system consists of 

 three pairs of ganglia, one pair near the oesophagus, one in the 

 foot, and one just under the large posterior muscle that pulls the 

 shell shut (Fig. 143). Pairs are connected with each other, and with 

 the body tissues by fibres. In the segmental invertebrates, Annelida 

 and Arthropoda, the ganglia occur in pairs, one pair in the ventral 

 body wall of each segment, except in the most anterior segments, in 

 which the ganglia are dorsal. In the earthworm, for instance (Fig. 

 144), there is a pair of ganglia dorsal to the pharyngeal cavity. 

 Nerve tracts connect these ganglia with a pair just under the 

 pharynx. The sub-pharyngeal ganglia are the most anterior pair 

 of a chain of segmental ganglia that are all connected with each 

 other and with the body tissues by nerve fibres. In addition, giant 

 fibres traverse the entire chain of ganglia. It is thought that these 

 giant fibres control the rapid contractions of the whole animal, 



