NERVOUS SYSTEM OF ARTHROPODA. 



253 



In addition to the nerves, set apart for the muscular system and 

 the integument, there are also nerves for the viscera ; of those the 

 enteric nerves are most exactly known. They partly agree with the 

 characters which obtain in the An- 

 nelid es. As separate ganglia are 

 embedded in their course, they con- 

 stitute to a certain extent an inde- 

 pendent nervous system, which is 

 called the ' c stomato-gastric nervous 

 system." Another special system, 

 of nerves, which obtains especially 

 in the Insecta, arises from the gan- 

 glia of the ventral medulla, and is 

 known as the sympathetic nervous 

 system. 



§ 196. 



The nervous system of the 

 Crustacea presents us with a large 

 number of examples of the phe- 

 nomena described in the foregoing 

 paragraph. The relation between 

 the size of the cerebrum and the 

 development of the optic organs is 

 seen in the Thoracostraca, and in 

 the large-eyed Hyperida (Phronima) 

 among the Arthrostraca, where the 

 optic nerves arise from distinct 

 lobes ; these lobes are also distinct 

 in the Wood-lice. The separation 

 of the cerebral mass into various 

 groups of ganglia generally implies 

 a higher differentiation. With this 

 ought to be compared those de- 

 generations which affect the cere- 

 brum when the optic organs are 

 reduced or altogether lost ; when 

 this happens the antennas also gene- 

 rally disappear. Conditions of this 

 kind are found in the parasitic Cope- 

 poda, and in the Cirripedia (Fig-. 

 129, B gs), and in consequence of it 

 the cerebrum is sometimes repre- 

 sented by nothing but a commissure. 



The most anterior of the ventral ganglia is connected to the 

 cerebrum by a shorter or longer commissure. The length of this 

 chord appears to depend on the position of the mouth with regard to 

 the cerebral ganglia (that is really to the eyes and antennas). In the 

 Malacostraca the length is very considerable (Fig. 128, c ; Fig. 129, A), 



Fig. 128. 

 Sqnilla. 



Nervous 

 Eyes. 



of 



system 

 First ; a" 

 Second pair of antennae, p Claws, 

 provided with prehensile terminal 

 joints, p 1 E owing- feet ; the last pair 

 of the foot. like appendages contribute 

 to form a caudal-fin. m Muscles. 

 r/s Supra-cesophageal ganglion. c 

 Commissural chords. g< Thoracic 

 ganglia, g" g'" g' y Abdominal ganglia. 



