20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



II. ONYCHOPHORA 



The Onychophora undoubtedly belong to the annelid-arthropod 

 group of animals ; their development is typically metameric and the 

 embryonic mesoderm contains a complete series of segmental coelomic 

 sacs. Certain features of their structure definitely separate the Ony- 

 chophora from the Arthropoda, but, on the other hand, several impor- 

 tant anatomical characters common to these two groups would appear 

 to relate the onychophorans more closely to the proarthropods than 

 to the annelids. 



The structural disparity between the Onychophora and the Arthrop- 

 oda is shown particularly in the nervous system and in the body 

 musculature. The longitudinal nerve cords of the Onychophora not 

 only do not form ganglia, but, instead of coming together as in the 

 annelids and arthropods, they move farther apart during their develop- 

 ment, and in the adult they lie laterad of a series of dorsoventral 

 muscles on each side of the body that would prevent their median 

 approximation. In the brain, the antennal nerve tracts run dorsal to 

 the optic lobes instead of ventral to them as in the arthropod brain. 

 The onychophoran body musculature consists of a continuous subepi- 

 dermal muscle layer composed of distinct sets of circular, oblique, 

 and longitudinal fibers, the arrangement of which in no way suggests 

 a possible primitive pattern of arthropod musculature. 



The relationship of the Onychophora to the Arthropoda is sug- 

 gested by the leglike segmental appendages of the latter, though the 

 onychophoran " legs " are little more than well-musculated pouches 

 of the body wall bearing each a pair of claws. It is in the evolution 

 of the mesodermal organs that are shown the most important features 

 common to the Onychophora and Arthropoda. In each group the 

 haemocoele is restored as the permanent body cavity as a result of 

 the reduction or complete dissolution of the coelomic sacs. The 

 nephridia of the Onychophora are coelomoducts, each with a remnant 

 of the corresponding coelomic sac attached to its inner end. The head 

 glands of Crustacea have the same structure as the onychophoran 

 nephridia, suggesting that they have had a common origin with the 

 latter, though they have, lost the vibratile cilia still retained in the 

 nephridial canals of the Onychophora. Finally, the reproductive 

 system of the Onychophora is decidedly arthropodan in its structure, 

 since the germaria (equivalent to the " gonads " of Annelida) are 

 enclosed in gonadial sacs derived from the coelomic walls, and the 

 gametes are discharged through gonoducts formed from a pair of 

 coelomic sacs and their outlets. The structure of the nephridial and 

 genital organs in the Onychophora and Arthropoda is, of course, a 



