320 THE INVERTEBRATA 



organ. The other part approaches its fellow in a mid-dorsal position 

 to form the heart lying between them (cf. Fig. 352 A) and while in the 

 anterior region it mostly disappears, those of the posterior segments 

 fuse longitudinally to form two tubes which become the gonads 

 (Fig. 217). 



At the same time the gaps between the organs become filled with 

 blood. A dorsal part of the haemocoele so formed is marked off by a 

 partition as the pericardium. This contains the heart, a long tube with 

 a pair of ostia in nearly every segment. There are, however, no other 

 blood vessels, so that the condition of the circulatory system is by no 

 means so advanced as in the higher Crustacea and the more primitive 

 arachnids. 



The possession of the perivisceral haemocoele almost diagnoses the 

 group as arthropods, but it was the discovery of the tracheae which led 

 to the inclusion of Pen]^«^w^ in that phylum. The stigmata are scattered 

 over the surface of the body most thickly on the sides and ventral sur- 

 face, several occurring in each segment. Each stigma leads into a pit, 

 penetrating the muscle of the body wall, from which arise bundles of 

 minute air-containing tubes which end in the various organs of the 

 body (Fig. 216 C). It can hardly be doubted that these tracheae are 

 definitely arthropodan in type : their most significant difference from 

 those of other forms is in their non-segmental character. Their 

 irregular distribution is only possible because they originate as pits in 

 soft skin; when once a cuticular exoskeleton has been established 

 tracheae can only be excavated in the joints between segments. 

 Probably then the Onychophora have never had a more definite 

 cuticle than they possess at present; if they had, tracheae have been 

 acquired since it was lost. 



The alimentary canal consists of short ectodermal fore gut and hind 

 gut and a very long endodermal mid gut, lined by a peritrophic mem- 

 brane (p. 434) which is thrown off periodically. The fore gut consists 

 of a buccal cavity into which open the large salivary glands and a mus- 

 cular suctorial pharynx. The mid gut possesses no separate glands. 



The excretory tubules (Fig. 216 B) are composed of a distal terminal 

 bladder, a coiled secretory canal and a ciliated duct which opens 

 into a much reduced coelomic vesicle. The bladder and probably the 

 whole of the canal are formed from ectoderm, the rest from meso- 

 derm. It can perhaps be said then that the tubule is a modified 

 coelomoduct which has attained its present condition by the tucking- 

 in of ectoderm at its external opening. The tubules form a complete 

 series, but some of them have been converted into uses other than 

 excretion. Thus the tubules corresponding to the oral papillae form 

 the salivary glands and are much larger and more complex than in 

 other segments. The anal glands and the gonoducts themselves have 



