PHYLUM ARTHROPOD A 



(509 



tr.o. 



enclosed by the circular lip raised up into a number of papilla- 

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

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

 a thick-walled 'pharynx (pilar.) leading to a narrow oesophagus. 

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

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

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

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

 wards from the buccal cavity receives the secretion of two long 

 narrow tubular salivary glands (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 

 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 

 tubes, 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, 

 co.r. 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. edwardsii — they 

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

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

 papillae, 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 brain (brn.) situated in the 

 head, and of two longitudinal nerve cords (ne. co.) which run parallel 



Flo. iS'2. — Section through a tracheal pit and diverg- 

 ing bundles of tracheal tubes of Peripatus. tr. 

 trachea; ; tr. c. cells in walls of trachea; ; tr. o. 

 tracheal stigma ; tr. p. tracheal pit. (From Camb. 

 Nat. Hist., after Balfour.) 



