ARTHROPODA. 101 



connective runs down on each side of the gullet to join an elon- 

 gated post-cesophageal ganglion, notched on each side, and repre- 

 senting the six ganglion-pairs of segments 3-8 inclusive. The 

 post-oesophageal ganglion is the first of the ventral chain, the 

 thoracic part of which lies in the sternal canal. The rest of 

 the cord consists of eleven well-marked pairs of ganglia united 

 by double connectives. The first five of these are thoracic ganglia 

 placed at unequal distances, the posterior ones being rather near 

 together. The connectives between the third and fourth diverge 

 to allow the sternal artery to pass between them. The last six 

 are the rather smaller abdominal ganglia placed at regular intervals 

 upon the ventral wall of the abdomen. The last is somewhat larger 

 than the others, and probably represents two pairs fused together. 



The ganglia supply nerves to their own segments. Two pairs 

 run from the cerebral ganglion to the eyestalks, and one pair each 

 to the antennules and antennae. The anterior part of the post- 

 cesophageal ganglion supplies the segments to which the mandibles, 

 maxillae, and first two pairs of foot-jaws belong. Its posterior 

 part (the only partially fused ganglion of the third thoracic 

 segment) supplies the third pair of foot-jaws, and behind it two 

 pairs of interganglionic nerves run to the other parts of the 

 segment. From the following ganglia two pairs of nerves arise 

 in most cases, the anterior of which supply the corresponding 

 pair of appendages. An interganglionic pair of nerves is found 

 (in the majority of segments) behind the ganglion. The sixth 

 abdominal ganglion supplies the last two segments. 



The visceral (sympathetic) nervous system consists of an 

 anterior visceral nerve (formed by the union of three trunks, one 

 running back from the cerebral ganglion, and one arising from 

 each oesophageal commissure), which branches out on the stomach, 

 and a posterior visceral nerve which runs forwards on the ventral 

 surface of the intestine from the last abdominal ganglion. Several 

 small ganglia are present on the branches of the anterior visceral 

 nerve. 



The ganglion-cells, which are here confined to the ganglia, 

 possess the usual parts, and are surrounded by special nucleated 

 sheaths. Each nerve-fibre is invested by a firm primitive -sheath, in 

 which are imbedded at irregular intervals, small nerve-corpuscles, 

 with large nuclei and scanty protoplasm, and within the sheath is 

 a clear axis-cylinder. 



