PHYLITBI ARTHROPODA 



(i09 



enclosed by the circular lip raised up into a number of papilke 

 bearing a few spines, and having on its roof a slight prominence, 

 the tongite, with a row of small spines or teeth. This is followed by 

 a thick-walled pharynx {pliar.) leading to a narrow (esophagus. 

 The part which follows, the mesenteron or stomach-intestine, a wide 

 somewhat thin-walled tube, extends nearly to the posterior end of 

 the body. The narrower redam leads to an anal aperture situated 

 on the last segment of the body. A diverticulum leading back- 

 wai'ds from the buccal cavity receives the secretion of two long 

 narrow tubular salivari/ glanJs (sal. gld.). 



Circulatory system. — The heart is an elongated tube run- 

 ning through nearly the entire length of the body. It presents a 

 number of pairs of ostia 



arranged segmentally — 

 i.e., one opposite each pair 



tr.o. 



Fig. 4.S'2. — Suction through a tracheal pit and diverg- 

 ing bundled of traclital tubes of FeripatuSi tr. 

 tracheal ; tr. c. cells in walls of trachea; ; tr. u. 

 traclieal stigma ; ti: p. tracheal pit. (tYom fti;ii6. 

 Nat. Hist. , after Balfour.) 



of legs. It is enclosed in 

 a pericardial sinus imper- 

 fectly cut off from the 

 general body-cavity by 

 a longitudinal partition. 

 The only other vessel is 

 a median ventral vessel. 



The organs of respir- 

 ation (Fig. 482) are de- 

 licate, unbranched or 

 rarely branched tracheal 

 tules, lined with a thin 

 chitinous layer exhibiting 

 fine transverse striations. 

 Groups of these open in 

 little depressions of the integument, the tracheal pits {tr.p.), the 

 external openings of which are known as the stigmata {tr.o.). The 

 stigmata in some of the species are distributed irregularly over 

 the surface; in others are arranged in longitudinal rows. By 

 means of these tubes air is conveyed to all parts of the body. 



A series of pairs of glands, the coxal glands (Fig. 481, 

 cox. gld.), lie in the lateral compartments of the body-cavity, and 

 their ducts open on the lower surfaces of the legs just outside the 

 nephridial apertures. Their distribution varies in the two sexes 

 and in the different species : in one species— P. ed%mrdsii—i\i.ey 

 are only developed in the male. A pair of larger glands— the 

 slime glands {si. gld.) — opening at the extremities of the oral 

 papillai, may be modified coxal glands: the secretion of these 

 is discharged in the form of a number of fine viscid threads when 

 the animal is irritated, and appears to serve a defensive purpose. 



The nervous system consists of a Irain {brn.) situated in the 

 head, and of two lungitiidinal neroe cords {ne. co.) which run parallel 



