284 THE INVERTEBRATA 



this is formed, and eventually is used to form the segmental excretory 

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

 while in the anterior region it mostly disappears, those of the posterior 

 segments fuse to form two longitudinal tubes which become the 

 gonads (Fig. 206). 



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

 blood. A dorsal part of the haemocoele so formed is marked oif 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 haemocoele almost diagnoses the group as 

 arthropods, but it was the discovery of the tracheae which led to the 

 inclusion oi Peripatus in that phylum. The stigmata are scattered over 

 the surface of the body most thickly on the sides and ventral surface, 

 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 fluid of the body cavity 

 (Fig. 205 C). It can hardly be doubted that these organs are definitely 

 arthropodan in type: their most significant difference from those of 

 other forms is in their non-segmental character. Their irregular dis- 

 tribution 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, which is ciliated. The fore 

 gut consists of a buccal cavity into which open the large salivary glands 

 and a muscular suctorial pharynx. The mid gut possesses no separate 

 glands. 



The excretory tubules (Fig. 205 B) are composed of a distal 

 terminal bladder, a coiled secretory canal and a funnel 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 com- 

 plete 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 them- 



