142 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



18. — Since modern Symphyla combine features of the progoneate 

 Diplopoda and Pauropoda on the one hand, and of the opisthogoneate 

 Hexapoda on the other, there can be httle question that they are direct 

 descendants of common ancestors of these two groups. Modern 

 Symphyla, however, are Hnked more closely with the progoneate 

 forms by the anterior position of the gonopore, the segmentation and 

 structure of the legs, and the retention of the movable laciniae of the 

 mandibles. The Protosymphyla, therefore, gave rise to an opistho- 

 goneate branch that became the Protohexapoda. 



In general appearance the Protosymphyla probably resembled their 

 modern representatives, but retained certain features of the Proto- 

 myriapoda that have been transmitted to the hexapod line, though lost 

 in the progoneate descendants. The legs were all alike and had the 

 /-segmented protomyriapod type of structure, but the coxopodites 

 bore each, mesad of the telopodite base, a small stylus and an eversible 

 vesicle, as in modern Symphyla (fig. 52 H),' which structures are pre- 

 served also on the abdomen of some of the apterygote insects (I) . The 

 appendages of the last body somite became reduced to styliform cerci. 

 The head appendages included a pair of antennae, a pair of mandibles, 

 and two pairs of maxillae. The lateral eyes must have been compound, 

 because compound eyes have been transmitted along the arthropod 

 line from the Trilobita to the Xiphosurida, the Crustacea, and through 

 the Protosymphyla to the Hexapoda. The protosymphylan mandibles 

 had the protomyriapodan structure, movable laciniae being well de- 

 veloped, and palpi absent. The first and second maxillae retained 

 the palpi and each acquired two basal lobes (lacinia and galea), fea- 

 tures transmitted to the hexapods, though the palpi have been lost in 

 the progoneate branch. The bases of the second maxillae, however, 

 became united to form a single appendage, the lahhim, an organ so 

 characteristic of all the descendants of the Protosymphyla that the 

 group as a whole, including Symphyla, Pauropoda, Diplopoda, and 

 Hexapoda, might well be designated the "Labiata" (fig. 54). 



/p. — The direct descendants of the progoneate branch of the proto- 

 symphylids are the modern Symphyla, but at an early period there 

 were evolved from the symphylid line the common ancestors of the 

 Diplopoda and Pauropoda. The Symphyla retain the generalized 

 structure of the body and appendages (fig. 52 A), but of the 16 body 

 segments evident in the dorsum of most forms, 3 are without appen- 

 dages. The legs (K) show the diplopod type of structure in the rela- 

 tively large size of the second trochanter {■^Tr) and the smallness of 

 the femur (Fin), but the coxae do not appear as typical leg segments, 

 since each pair apparently is confluent in a large posterior division of 



