TYPE TRACHEATA. 477 



tition. A pair of ostia are situated ou the dorsal surface of 

 the heart in each inetaroere, and pass the blood into the heart 

 from the pericardia! space. Respiration is performed by 

 tracheae (Fig. 219, tr) consisting of slender unbranched tubes 

 which arise in bunches from stigmata, either scattered ir- 

 regularly over the surface of the body in considerable 

 numbers or else arranged, as in P. capensis, somewhat imper- 

 fectly in two rows upon the dorsal and two on the ventral 

 surface of the body. 



The mouth opens into the mouth-cavity containing the 

 mandibles, and this communicates posteriorly with a muscu- 

 lar pharynx, and has opening into it the ducts of two long 

 tubular salivary glands (sp) which extend through more than 

 half the length of the body. The pharynx (p) communicates 

 by a short oesophagus with the stomach, which extends as a 

 straight tube almost to the extremity of the body, where a 

 short rectum places it in connection with the anus. The 

 pharynx and oesophagus and the rectum are lined with chitin 

 and represent the fore-gut and hind-gut of other Tracheates, 

 the stomach being the mid-gut. No Malpighiau tubules or 

 other diverticula of the intestine occur. 



The nervous system shows several highly-interesting 

 features. There is, as is usual in metameric animals, a supra- 

 cesophageal ganglion-mass (Fig. 219, og) composed of at least 

 two and probably three pairs of ganglia, of which the first 

 supplies the antennae and the second the mandibles, while a 

 third pair lies at the sides in close contact with the second 

 pair and sends nerves to the oral papillae. These latter are, 

 however, postoral and ventral in position, and from them 

 there extend back two ventral cords (bni) which in each 

 metamere dilate into a gaugliouic swelling. The two ventral 

 cords are, however, widely separated, lying in the lateral 

 chambers of the coelom (Fig. 220), and are connected by a 

 large number of cross-commissures a condition which recalls 

 the arrangement in the Amphiueurous Mollusca (see Fig. 

 124), the similarity being further increased by the facts that 

 the two cords unite behind and above the rectum, as in the 

 Solenogastres, and that the ganglion-cells are not confined to 

 the enlargements but are scattered all along the cords. The 



