228 ELEMENTARY BIOLOGY. [CHAP. 



€. The sicboesophageal ganglion ; a large oval mass, 

 situated immediately behind the gullet. It dis- 

 tributes nerves to the mandibular and five following 

 somites. 



/. The thoracic ganglia; five pairs, each somewhat 

 larger than the abdominal, distributing nerves to 

 the five posterior thoracic somites. 



a. The posterior pair ; situated in firont of their cor- 

 responding somite, and consequently far removed 

 from the first abdominal pair. This is seen to 

 be the result of approximation towards the pair 

 in front, the two lying close together immediately 

 behind the passage for the sternal artery. 



ft. The second a?id third pairs; similarly but less 

 conspicuously approximated, in front of the 

 artery. 



y. The anterior pair ; equidistant between ft and 

 the subcesophageal ganglion. 



2. The nenfes of the supraoesophageal ganglion. Carefully 

 remove one half of the rostrum and lay open the 

 eye-stalk of the same side. Note, passing into the 

 latter — 



a. The optic nerve ; a thick tract, arising from the 

 anterior end of the ganglion and expanding within 

 the eye-stalk to form the optic bulb. 



b. The antennulary nerve; a delicate trunk, arising 

 from the outer border of the ganglion ; it passes 

 outwards and forwards to enter the base of the an-< 

 tennule. 



c. The antennary nerve ; a similar trunk, arising from] 

 the hinder region of the ganglion; it passes back- 



