STRUCTURE OF THE COCKROACH. 277 



The skin consists of an external chitinous cuticle and a 

 subjacent cellular layer — the epidermis or hypodermis — from 

 which the cuticle is formed. The newly hatched cockroaches 

 are white, the adults are dark brown. Moulting, which in- 

 volves a casting of the cuticle, of the internal lining of the 

 tracheae, etc., occurs some seven times before the cockroach 

 attains in its fifth year to adult maturity. 



The muscles, which move the appendages, and produce 

 abdominal movements essential to respiration, are markedly 

 cross-striped. 



Nervous System. — A pair of supra-cesophageal or cerebral 

 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 connected 

 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 three pairs in the 

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

 pair are largest, nerves are given off to adjacent parts. From 

 the oesophageal commissures two visceral nerves are given 

 off, which form in a somewhat complex manner the-innervation 

 of gullet, crop, and gizzard. Besides the large compound eyes, 

 there are other sensory structures — some of the hairs on the 

 skin, the maxillae (to some extent organs of taste), the 

 antenna (tactile and olfactory), the anal cerci (tactile), and 

 possibly the oval white patches on the head. 



Alimentary System. — (t) The fore-gut (stomatodaeum) 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 ; {b) of the 

 narrow gullet or oesophagus ; (c) of the swollen crop ; (d) of 

 the gizzard or proventriculus with muscular walls, six hard 

 cuticular teeth, and some bristly pads. 



There is a pair of diffuse salivary glands on each side of 

 the crop, and between each pair of glands a salivary recep- 

 tacle. The ducts of each salivary gland unite on each side, 

 the two ducts thus formed unite 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. 



