374 



PHYLUM ARTHROPODA 



Nervous system. — A pair of supra-oesophageal or cere- 

 bral ganglia lie united in the head. As a brain they receive 

 impressions by antennary and optic nerves. By means of a 

 paired commissure surrounding the gullet, they are con- 

 nected with a double ventral chain of ten ganglia. Of 

 these, the first or sub-oesophageal pair are large, and give 

 off nerves to the mouth-parts, etc. ; from each of the 

 ganglia of the thorax and the abdomen nerves are given off 

 to adjacent parts. There are three pairs of ganglia in 

 the thorax, and six in the abdomen, of which the last is 

 the largest. From the oesophageal commissures visceral 

 nerves are given off to the gullet, crop, and gizzard. 



Besides the large compound 

 eyes, there are other sensory 

 structures — -some of the setae 

 on the skin, the maxillae (to 

 some extent organs of taste), the 

 antennae (tactile and olfactory), 

 the anal cerci (tactile), and 

 possibly the oval white patches 

 on the head. 



Alimentary system. — (i) 

 The fore-gut (stomodaeum) is 

 lined by a chitinous cuticle 

 continuous with that of the 

 outer surface of the body. It 

 includes — (a) the buccal or 

 mouth cavity, in which there is a tongue-like ridge, and 

 into which there opens the duct of the salivary glands ; 

 (^) the narrow gullet or oesophagus ; (c) the swollen 

 crop ; {d) the gizzard, with muscular walls, six hard 

 cuticular teeth, and some bristly pads. 



Fig. 212. — Leg of cockroach. 



c, Broad expanded coxa; tr., troch- 

 anter; /..femur; ti., tibia; /a., six- 

 jointed tarsus with terminal claws 

 and adhesive cushions. 



There is a pair of diffuse sahvary glands on each side of the crop, and 

 between each pair of glands a salivary receptacle. The ducts of the 

 two salivary glands on each side unite ; the two ducts thus formed 

 combine in a median duct, and this unites with another median duct 

 formed from the union of the ducts of the receptacles. The common 

 duct opens into the mouth. 



(2) The mid-gut (mesenteron) is lined by endoderm. It 

 is short and narrow, and with its anterior end seven or 



