408 



AETIIEOPODA. 



aii approximation or fusion of the corresponding ganglia has taken 

 place. In one case only, viz., in the Pentastomidce, which in form 

 and grade of life resemble the intestinal worms, the dorsal part of 

 the oesophageal commissure is not swollen out to form a cerebral 

 ganglion, and the central parts of the nervous system are com- 

 pressed together into a common gangli- 

 onic mass beneath the 03sophagus. In 

 all other cases the brain is a large gangli- 

 onic mass lying above the oesophagus? 

 and connected by means of the cesophageal 

 ring with the anterior ganglion of the 

 ventral chain, which is usually placed in 

 the head and is known as the suboeso- 

 phageal ganglion (fig. 328). The sense 

 nerves arise from the brain, while the 

 ganglia of the ventral chain send nerves 

 to the muscles, organs of locomotion and 

 the body covering. 



Visceral nervous system. In addition 

 to the brain and ventral ganglionic chain, 

 which are comparable to the cerebro- spinal 

 system of Vertebrata, we can distinguish 

 in the larger and more highly organised 

 Arthropods a visceral nervous system 

 (sympathetic], which consists of special 

 ganglia and plexuses connected with the 

 other system and specially distributed to 

 the alimentary canal. In the higher Ar- 

 thropoda, paired and unpaired visceral 

 nerves are very generally present, both 

 of which have their origin in the brain. 

 Sense organs. Eyes are most generally 

 FIG. 328. Nervous system of distributed, and are only absent in a few 



paragitic f orms . J n their simplest 'form 



r _ " 



they are paired or unpaired structures 



p ia ~ d p n the bi - ain ' ^-^ with re - 



fractive bodies, and with or without a 



\v 



the larva of Coccinella (after 

 Ed. Brandt,). Ofr, Frontal 



ganglion ; G, brain ; Sg, sub- 



chain in the thorax and 



simple lens (stemmata, or simple eyes). 

 The compound eyes, which are always paired, are much more 

 complicated. They are distinguished by the presence of nervous 

 rods and crystalline cone?-, and may be divided into faceted eyes 



