70 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



THE RESPIRATORY ORGANS 



The Onychophora are provided with numerous fine tubular in- 

 growths from the body wall, which undoubtedly serve for respiration, 

 and are therefore termed tracheae, though it is possible that ana- 

 tomically they are more of the nature of insect tracheoles. The 

 tubules, which are only one to three microns in diameter, arise in 

 dense bundles (fig. 32 B, Tra) from small flask-shaped pits (tp) of 

 the integument, and extend long distances into the body cavity. The 

 tracheal pits may be very numerous ; they occur on all parts of the 

 body, on the head, and around the mouth, but they are most abundant 

 on the back, where several may occupy the space of a square milli- 

 meter. For the most part the pits are irregularly distributed, but in 

 some species they are arranged in longitudinal rows. The tracheal 

 bundles issuing from the inner ends of the pits contain large, con- 

 spicuous nuclei in their basal parts (Nu), which probably pertain to 

 the matrix cells, but the tubes themselves diverge and extend far 

 beyond these nuclei. According to Dakin (1920), the tracheal walls 

 are strengthened by excessively minute but perfect spiral fibers visible 

 in fresh material. In their distal parts the tracheae are branched and 

 go to practically all the internal organs, but their final terminations 

 have not been observed. 



Since tracheal invaginations of the body wall are developed for 

 respiratory purposes in nearly all groups of terrestrial arthropods, 

 the mere presence of such organs can have no taxonomic significance, 

 any more than has the presence of gills in diverse groups of aquatic 

 animals. Inasmuch as invertebrates breathe through the skin in any 

 case, evaginations or invaginations of the integument are about the 

 only devices they can develop for improving their respiratory 

 functions. 



THE CIRCULATORY SYSTEM 



The blood vascular system of the Onychophora consists only of a 

 tubular dorsal vessel (fig. 29, DV) extending the entire length of 

 the body, said to be open anteriorly and posteriorly. The walls of 

 the vessel consist of circular muscle fibers, and are perforated dorsally 

 in each segment by a pair of ostia. The tube is suspended from the 

 body wall by connective tissue strands, and is supported on a mem- 

 branous and muscular dorsal diaphragm (DDph). The diaphragm 

 muscles are fine, regularly transverse fibers medially attached on the 

 ventral wall of the blood vessel ; laterally they penetrate between the 

 fibers of the dorsal somatic muscles and are apparently attached on 



