COMMON COCKROACH. 143 



frontale, triangular in shape, lying in front of the supra-oesophageal ganglion, and 

 giving off posteriorly a nervus recurrens, which courses along the dorsal wall of 

 the oesophagus and crop ; (2) of a couple of paired ganglia, lying on either side of 

 the nervus recurrens anteriorly, and connected to the under side of the supra- 

 oesophageal ganglion and to the nervus recurrens; (3) of a triangular ganglion 

 terminating the nervus recurrens, from which two nerves, one on either side, run 

 obliquely down the walls of the crop, and end in the muscular walls of the gizzard. 

 Each nerve, near its termination, has a small ganglionic enlargement. There is 

 also a sympathetic system in connection with the ventral chain. A fine nerve 

 springs from either the right or left commissure, connecting successive ventral 

 ganglia, and about midway between the ganglia. It runs dorsally between the 

 commissures, and just above the ganglion behind its place of origin it divides. 

 Each branch swells into a. long spindle-shaped ganglion, and then joins the lateral 

 nerve of the same side coming from the ventral ganglion. 



With the exception of the chylific stomach, the digestive tract is lined by a 

 chitinous cuticula, which is for the most part beset with setae. The chitinous coat 

 is continued into the salivary ducts and receptacles. In the ducts it is striated, as 

 in the tracheae (infra). The finest branches of the ducts in the acini of the gland 

 are smooth-walled, and their terminal dilatations are lined by a coat which is not 

 chitinous. The walls of the digestive tract consist of an external membrane under- 

 lain by an outer layer of circular, and an inner layer of longitudinal 1 striated 

 muscle-fibres. Then follows a layer of connective tissue cells, and a layer of 

 columnar cells, which secrete the internal cuticula where it is present. The chylific 

 stomach is lined by columnar cells, with a striated border and rounded glandular 

 cells lying in depressions. The gizzard has on its internal surface six longitudinal 

 projections or teeth, and behind each tooth two cushions, the first with an uneven, 

 the second with an even surface. In the interval between two teeth are three 

 parallel folds, and between each of these and the teeth a smaller fold. The 

 posterior part, which projects into the chylific stomach, contains six principal folds 

 in a line with the cushions, and between each of them a small accessory fold. The 

 circular muscle layer is strongly developed, and according to Wilde, longitudinal 

 fibres run from the teeth to the cushions, and on the outer surface of the posterior 

 part, whilst radial fibres are found only in the anterior and posterior parts. The 

 same authority states that in some Orthoptera the internal cuticula is cast off at each 

 moult. According to Krukenberg (Untersuchungen Physiol. Inst. Heidelberg, ii. 

 1882, p. 26), the salivary glands secrete a purely diastatic ferment, the chylific 

 stomach both diastatic, peptic and tryptic ferments. The ridges in the rectum 

 consist of elongated hypodermis cells, underlain by a mass of connective tissue 

 cells, richly supplied with tracheae. They are structures highly developed in the 

 larvae cf Dragon-flies. These animals take in and expel water from the rectum, 

 which thus becomes an important respiratory organ. 



The Malpighian vessels consist of an outer homogeneous layer, a single layer 



planeta, immediately beneath the ventral nerve-chain. These cells eventually ensheath the nervous 

 structures. Nussbaum suggests that the ridge represents the chorda supraspinalis in Lepidoptera ; cf. 

 infra, p. 160. 



1 There is pparently a contradiction in Basch's paper (cited below) as to the position, &c. 

 of these 1 yers. Cp. pp. 241, 249, 252 of his paper. 



