PHYLUM ARTHROPODA 



507 



ORDER 1. SYMPHVLA. 



Myriapoda in which there are not more than twelve leg-bearing 

 trunk segments ; a single pair of branching tracheae, the external 

 apertures of which are situated in the head ; and an unpaired 

 genital aperture on the fourth segment. 

 - This order includes only a single 

 genus, Scolopcndrclla (Fig. 448.) 



ORDER 2. CHILOPODA. 



Myriapoda with a dorso-ventrally 

 compressed body, consisting of a con- 

 siderable number (15 to 173) of trunk 

 segments, each with a single pair of 

 legs, the first pair of legs forming poison 

 jaws (maxilHpedes). The unpaired 

 genital aperture is on the second last 

 segment. 



This order includes the Centipedes 

 (Fig. 449) and Scutigera. 



ORDER 3. DIPLOPODA. 



Myriapoda with a dorsally convex 

 body composed of a considerable num- 

 ber of segments, all of which, with the 

 exception of the first four, bear each 

 two pairs of legs. There are no 

 maxillipedes. The paired genital aper- 

 tures are situated between the third and fourth segments. 



This order includes the Millipedes. 



ORDER 4. PAUROPODA. 



Myriapoda with ten trunk segments and nine pairs of legs. 

 Antennae with several flagella. Tracheae not known. Paired 

 genital apertures situated at the bases of the second pair of legs. 



*The order includes only a single genus, Pauropus (Fig. 451). 

 GENERAL ORGANISATION. 



External features. The head in the Myriapoda is as well 

 marked off as in an Insect ; it appears to be composed of about 

 four fused segments. The antennce consist sometimes of many, 

 sometimes of comparatively few segments. A pair of eyes, 

 situated on the dorsal surface of the head, consist of aggre- 

 gations of ocelli except in Scutigera, in which there are com- 

 pound eyes, differing, however, in their structure from those 

 of Insects. 1 There is a movable lalrum, a pair of mandibles, and 



1 See p. 596. 



FIG. 448. Scolopendrella 



immaculata (From Leuck- 

 art, after Latzel.) 



