498 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



A review of the evidence now available leads one to the conclusion that the 

 Pauropoda, Diplopoda, Chilopoda, Symphyla, CoUembola, Protnra, Entotrophi, 

 Thysanura, and Pterygota are a series of forms with a common ancestor not 

 shared with any otlier Arthropoda. Furthermore, the discontinuities between 

 adjacent members of this series may seem large when one or two characters are 

 considered, but always important linking characters complicate the establish- 

 ment of classificatory groupings. The distribution of each of the characters be- 

 lieved to carry the most weight in phylogenetic reasoning can be summarized 

 briefly. 



1. Cleavage: Early cleavage is entire in the Pauropoda, Diplopoda, Chilopoda, Sj'm- 

 phyla, and Collembola. It is incomplete in the Entotrophi, Thysanura, and Pterygota. 

 The embryonic development of the Protura is unknown. 



2. Evihryonic Membranes: Amnion and serosa cover the entire embryo of Pterygota 

 (rarely secondarily lost) and all but a small pore in the Thysanura. The Collembola, all 

 four myriapod classes, and the Entotrophi lack these embryonic membranes. 



3. Tiegs' Organ: This embryonic "dorsal organ" is absent in Pterygota, Thysanura, 

 and Pauropoda and present in Collembola, Symphyla, and Entotrophi. The situation for 

 embryos of Diplopoda and Chilopoda is not known. 



4. Evihryonic Yolk-Cells: Pauropoda, Diplopoda, and Symphyla have no segregation 

 of vitellophags and tissue-forming cells. The other five groups whose embryonic develop- 

 ment is known have separate vitellophages. 



5. "Pupoicl Stage": In the Pauropoda, Diplopoda, Chilopoda, and Collembola the em- 

 bryo emerges precociously and undergoes an inactive phase within the embryonic cuticle. 

 This stage does not exist in Symphyla, Entotrophi, Thysanura, or Pterygota. 



6. Anamorphosis: Pterygota, Thysanura, Entotrophi, Collembola, and some Chilopoda 

 are epimorphic, that is, body segments are not added during postembryonic development. 

 In the Pauropoda and Diplopoda several segments and pairs of legs are added during 

 development; usually the first instar has only three pairs of legs! The Protura emerge 

 from the egg with twelve postcephalic segments and pass through four pre-imaginal stages 

 resulting finally in an adult with 1.5 postcephalic segments (Tuxen, 1949). Symphyla 

 eclose with six or seven pairs of legs and nine postcephalic segments, and after a series 

 of ecdyses the imaginal state is reached with twelve pairs of legs and fourteen post- 

 cephalic segments. Some Chilopoda are anamorphic. 



7. Antennae : The apparent segments have their own intrinsic musculature in the 

 Collembola, myriapods, and Entotrophi. There is no segmental musculature in the fiagel- 

 lum of the Thysanura and Pterygota. The Protura lack antennae. 



8. Gnathocephalon : In the Pauropoda and probably Diplopoda there are only two 

 gnathal segments — the mandibular and first maxillary; the first maxillae are fused as 

 a complex gnathochilarium. The seven other groups have three gnathal segments. In the 

 Chilopoda the first maxillae are fused as a sort of labium, but in the other six groups the 

 second maxillae are fused as the labium or lower lip of the pre-oral cavity, and the sepa- 

 rate first maxillae, like the mandibles, move freely in the cavity. The mandibles have 

 two points of articulation to the cranium in Lepismatoidea and Pterygota; in the Machil- 

 oidea, Entotrophi, and the others, there is a single point of articulation. 



9. Tagmosis: The first three postcephalic segments are grouped into a thorax dis- 

 crete from the other segments (abdomen), and only these bear the legs in the Collembola, 

 Protura, Entotrophi, Thysanura, and Pterygota (hence — "Hexapoda"). In the four myria- 

 pod groups there is no thoracic-abdominal tagmosis. The hexapodous condition is not 

 necessarily a sign of common ancestry; hexapodous Acarina are well known, and the first 

 instar nymph of Diplopoda and Pauropoda is hexapodous and superficially resembles 

 Collembola and larvae of some Pterygota. 



10. Gonopore Site: In the Chilopoda, Collembola, Protura, Entotrophi, Thysanura, 

 and Pterygota the genital opening is posterior. In the Pauropoda, Diplopoda, and Sym- 

 phyla it is anterior. However (see above), Tiegs has shown that, at least in the Symphyla 

 and Pauropoda, the progoneate condition must not be given too much weight. It need 

 not imply common progoneate ancestry for these three groups. 



