NERVOUS SYSTEM. 47 



siderably the largest and most massive ; it is squarish, 

 or oval, or heart-shaped; it presents no trace of being 

 formed by the union of two lateral ganglia. Two great 

 nerves spring from its under side (a), represented in the 

 woodcut on page 49, by dotted lines), and run straight 

 down amongst the viscera in the prosoma : these nerves 

 are about as large as those forming the collar and those 

 running to the second ganglion ; hence, six great nerves 

 meet here, two in front, two behind, and two on the 

 under side. At the anterior end, over the junction with 

 the collar chord, three equal-sized nerves rise on each side, 

 with a fourth, smaller one, outside; these go to the 

 tropin and to the two olfactory sacks. At the posterior 

 end, on each side, a pair of nerves branch out rectangu- 

 larly, one of which (a,) goes to the first cirrus, and there 

 divides into two branches; of these, the upper runs up 

 the cirrus, and the lower one downwards. The other 

 nerve ($), proceeding on each side from this first thoracic 

 ganglion, runs to the muscles beneath the basal articula- 

 tion of the first cirrus. The collar surrounding the oeso- 

 phagus is generally very long, sometimes equalling the 

 whole thoracic chord ; at a middle point, a small branch 

 is sent off, and at the anterior end (e, e), close to the 

 supra-cesophageal ganglia, double or treble fine branches 

 run to the true ovaria, lying close to the upper end of the 

 stomach. The four (or only three) other thoracic ganglia, 

 when viewed as transparent bodies, are seen to be solid ; 

 but in some of the genera, as in Conchoderma, the outline 

 plainly shows, that each consists of a lateral pair fused 

 together. The second thoracic ganglion (b) is rather 

 small; it is either close to the first, as in PoUicipes 

 mitetta and Lep as fascicular is, or far distant, as in Ibla. 

 The third (c) and fourth are of about the same size with 

 the second : these three ganglia send large branches to 

 the second, third, and fourth pair of cirri : other minute 

 branches spring from their under sides, and from the 

 intermediate double chords. The fifth ganglion is larger 

 and longer than the three preceding ones, and gives off 



