146 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



one family. The paired genital ducts in both sexes open on the 

 eleventh abdominal segment. The CoUembola are the most aberrant 

 of all the hexapods, and in some ways the most primitive. They 

 have only nine body segments, and the single genital opening is on 

 the fifth abdominal segment. There can be no question that the Col- 

 lembola are derived from more generalized ancestors having a greater 

 number of segments, but since, in their phylogenetic history, segment 

 formation in the zone of growth has ceased after the establishment 

 of the genital ducts in the eighth somite, it is fruitless to look for 

 evidence of the ancestral segmentation in the embryogeny of present- 

 day CoUembola. The three pairs of appendicular organs on the 

 collembolan abdomen are unique in structure, and give little suggestion 

 of homology with the abdominal appendages of Symphyla, Diplura, 

 and Thysanura, though it may be supposed that the collophore is a 

 pair of united eversible vesicles, and that the two paired appendages 

 are highly developed styli. (For a fuller discussion of the special 

 features of the CoUembola, see Imms, 1936.) 



2^. — The main evolutionary line of the early hexapods led from 

 the opisthogoneate branch of the Protosymphyla directly into the 

 Machilidae, since in this family are best preserved the coxal accessory 

 structures of the symphylids (fig. 52 I) along with the normal ecto- 

 gnathous mouth parts. Moreover, it was in the ancestors of the 

 Machilidae that the characteristic ovipositor of the hexapods had its 

 inception, and, therefore, from the machilid line have been evolved 

 the Lepismatidae and the Pterygota. The common ancestors of these 

 last two groups developed two special features in the head structure. 

 One was the acquisition of a secondary anterior articulation of the 

 mandible on the cranium, giving the jaw a hinge movement on a longi- 

 tudinal axis, which brought about a reorganization of the mandibular 

 musculature, giving the principal function of abduction and adduction 

 to the dorsal muscles, and reducing the ventral muscles to a condition 

 of such little importance that they have completely disappeared in 

 the higher Pterygota. The other feature was the development of the 

 endocranial framework known as the tentorium, characteristically 

 present in Lepismatidae and Pterygota, but foretokened in Machilidae. 

 The tentorium is evidently a product of the hypopharyngeal apodemes 

 and of a transverse bar developed in the back of the head from lateral 

 invaginations. Both structures are present in Machilidae, but are 

 not united. In Lepismatidae the anterior apodemes are reflected 

 directly from the cranial margins and are united posteriorly with the 

 transverse bar, producing a typical tentorium. In the Pterygota the 

 roots of the anterior arms take a submarginal position on the cranium, 

 and in higher forms they have migrated to the facial aspect of the head. 



