156 PACKARD—CLASSIFICATION OF ARTHROPODA. ([April3, | 
ing those behind them, so as to give rise to the anomalous double 
segments ; the feet arise close together along the median line of the 
body, there being no sternal plates between them. In the typical 
Diplopoda the head consists of three segments, the preoral or an- 
tennal and two postoral, bearing the mandibles (protomalz) and 
the single pair of maxillz (deutomale) united to form the gnatho- 
chilarium or underlip. As all the members of this phylum agree in 
having from two- to three-jointed mandibles, in which respect they 
differ from Chilopod Myriopods and especially insects, we have 
given the name JZerogoda to this phylum in allusion to the primi- 
tive nature of these appendages, which resemble the maxillz rather 
than the mandibles of insects.1_ The mandibles of the Diplopods 
consist of three segments, a basal segment (cardo), a middle seg- 
ment (stipes), and a distal one (mala mandibularis), which sup- 
ports two lobes homologous with the galea and lacinia of the — 
maxilla of an insect. Diplopods are also provided with eversible 
coxal glands, in position like those of Scolopendrella ; these perhaps } 
functioning as blood-gills, and in Lysiopetalum occurring between 
the coxe of the third to sixteenth pair of limbs. | 
A primitive feature, and the one diagnostic of the Meropodaas 
compared with the Chilopodous Myriopods, is the paired genital j 
ducts and openings which are situated near the head between the 
second and third pair of legs. In the Symphyla the opening is 
single, proving the later origin of that group. Another diagnostic 
feature is that the male genital glands lie beneath, while in Peri- 
patus, Chilopods and insects they lie above the digestive canal. 
The tracheary system is also more primitive than in Chilopoda | 
and insects, the trachez not being branched (except in Glomeridz) . 
and anastomosing, and the tracheze themselves are without spiral 
threads (tenidia). In Diplopods the stigmata, which are per- 
manently open, are placed beneath the legs, oreven in the coxal 
joints. The nervous system is much more primitive than in Chilo- 
poda and insects. The external genital armature, a complicated 
apparatus of male claspers and hooks, apparently arises from the 
sternum of the sixth trunk-segment, and they are modifications 
of the seventh pair of legs. F 
In their embryology the Diplopoda are more primitive than the 
1 There is an approach to this trimerous condition in Thysanura and Orthop- 
tera (Blatta) and certain Coleoptera (see my Zéext-Book of Entomology, p. 60, 
also p. 12), 
