NERVOUS SYSTEMS OF INVERTEBRATES 



ii 



In the higher members of the group, e.g. lobsters and insects, the 

 segments in front have fused into a well-developed head, followed 

 by a thorax, to constitute which other segments have coalesced, 

 while this is succeeded by an abdomen, where the amount of union 

 of segments varies greatly in different cases. These three suc- 

 cessive regions of the body differ greatly from one another as to 

 size and shape, and may undergo further fusion. Thus, in a 



A. 



HEADSLI 



PROBOSCIS PORE 



LATERAL NERVE 

 PROTRUDED PROBOSCIS 



LATERALNERVE 



GUT 



Fig. 1013. Diagrams to illustrate Structure of a Nemertine Worm, represented as a transparent object 

 A., Side view; B., front end, seen from above; c., cross section. 



Lobster, head and thorax are welded together, and in a Spider 

 not only is this so, but the abdominal segments have closely united 

 into a rounded mass. 



The nervous system of an Arthropod, like that of an Annelid, 

 consists of nerve-ring and double ventral nerve-cord, but the 

 ganglia are better developed, and in the higher members of the 

 group they are more or less fused together into larger nerve- 

 masses, just as the segments to which they belong are similarly 

 united. There is, in other words, an increasing amount of cen- 

 tralization in the nervous system as we pass from lower to higher 

 forms in any subdivision of the Arthropods. And this is clearly 



