462 ONlClIOrUOEA 



Key to the divisions of Tracheata: 



Oi Body worm-like, with a large number of pairs of legs. 



61 Body not externally segmented ; animals tropical and subtropical. 



1. Onyciiopiiora 



5a Body distinctly segmented 2. Myriapoda 



aa Body divided into head, thorax, and abdomen ; 3 pairs of legs. . . .3. Insecta 



Division 1. ONYCHOPHORA.* (Protracheata.) 



Primitive, worm-like tracheates which combine annelid with tracheate 

 characters. The surface of the body does not show segmentation, but is 

 annulated, and from its ventral surface projects a series of pairs of short 

 annulated ajDpendages, which in the various species may number from 

 14 to 43 pairs. The head bears a pair of simple eyes, a pair of segmented 

 antennae, a pair of hooked jaws surrounded by a circular lip, and a pair 

 of short leg-like oral papillae in which are the apertures of a pair of slime 

 glands; the sticky secretion of these glands serves to catch insects and 

 spiders. Coxal glands, of which there are as many pairs as legs, are also 

 present. On the under surface of each leg also is a conspicuous elongated 

 groove which is the opening of a nephridium. The skin is soft and 

 thickly beset with papillae. 



The body cavity resembles that of insects in being an extensive blood 

 space and not a true coelom. The digestive system consists of a straight 

 tube passing from the mouth to the anus at the hinder end of the body; 

 the only digestive glands are a pair of long salivary glands. The heart 

 is a long muscular tube with segmental, paired openings. The respiratory 

 system consists of bunches of short tracheae, each of which opens through 

 a spiracle to the outside, and which are scattered iiTegularly above and 

 between the legs. The excretory system consists of a pair of nephridia at 

 the base of each pair of legs. Each nephridium opens into a vesicle 

 which is a remnant of the coelom, and externally in the elongated groove 

 on the under surface of a leg. The nervous system consists of a dorsal 

 brain and of a pair of ventral chords which extend from the brain the 

 length of the body and unite behind the anus ; they are joined by numer- 

 ous transverse commissures. The sexes are separate, the genital opening 

 lying in front of the anus. The gonads are paired, their ducts being 

 modified nephridia. Some species are viviparous and some lay eggs. In 

 many forms the egg contains much yolk and segments superficially, as is 

 the case in insects, while in others it is poor in yolk and segmentation is 

 total, and the embryo is enclosed in folds of the uterus 'and nourished by 

 means of a placenta. 



* See "Monographio des Onychophores," by E. L. Bouvier, Ann. des Scl. Nat., 

 Zool., 1907. "Tlie Present Distribution of the Onychophora," etc., by A. 11. Clark, 

 Smithsonian Misc. Coll., Vol. 65, 1915. 



