Characteristics of Myriopoda. cxv 



The Class Myriopoda is ordinarily divided into two orders— the Chilo- 

 poda and the Chilognatha or Diidopoda, from which the Si;phonizaniia 

 and the genus Pauroims ought, it is probable, to be separated. 



The Chilopoda are the most highly organized of the Myi-iopoda. They 

 are distinguishable externally by the flatness of their bodies ; the large 

 size of their antennae, Avhich always possess fourteen joints at least ; by 

 the modification of the two anterior pairs of post-cephalic appendages 

 into foot-jaws, the hinder pair of which is armed with a sickle-shaped 

 unguis and poison-gland ; and by their locomotor legs being attached in 

 ■single pairs. With these external characters, the following points of 

 internal structure are correlated ; the stigmata for the admission of air 

 to the tracheae do not correspond with the number of segments, and are 

 situated on the sides of the body between the bases of the feet and the 

 dorsal shields ; the generative ducts open at the posterior extremity of 

 the body ; and there is no intromittent organ. It is amongst the Chilo- 

 X:>oda only (in Scutigera) that we meet with compound facetted eyes. In 

 another Chilopodous family {Scolopendridae) we find the generative ducts 

 single, and the tracheae anastomosing as in Insects. The larval Chilo- 

 poda may have as many as six or eight pairs of locomotor appendages 

 when they leave the ^g% ; and the addition of fresh segments takes place 

 in the way of intercalation at each moult, in the intervals between each 

 pair of older segments. 



In the Chilognatha the body is sub-cylindriform, the antennae are 

 inconspicuous and do not possess more than seven joints ; the two 

 anterior pairs of post-cephalic appendages, or at least the first of 

 them, may be spoken of as 'foot-jaws,' or as forming 'labia/ inas- 

 much as they are directed forwards like the operculiform 'foot-jaws' 

 of Crustacea, and have their basal joints or coxae more or less enlaro-ed, 

 apposed mesially, and anchylosed, though they are much less altered in 

 function and structure than their homologues in the Chilopoda; and 

 their legs are, after the first six, or, in the males, seven post-cephalic 

 segments, arranged in double pairs. Differing thus externally from the 

 Chilopoda, they differ from them also in the following points of structure 

 and developmental history, and approximate more closely than they do 

 to the Arachnida and Crustacea, and less closely to the Insecta. The 

 stigmata leading into their tracheae correspond in number with their 

 segments, and are situated on the anterior border of the ventral plates, 

 under cover of the coxae of the legs, which are articulated to the pos- 

 terior border of these plates ; the tracheae do not anastomose ; the gene- 

 I'ative ducts are bilaterally symmetrical, having double openings in or 

 upon the borders of the third thoracic segment, which never carries legs, 



h% 



