NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS I45 



dition in the Hexapoda is not comparable with that in the Chilopoda, 

 in which the genital outlet, though always subterminal, may be on a 

 quite different somite in different forms because of the variable 

 number of somites that may precede it. There is reason for believing, 

 therefore, that the opisthogoneate condition of the Hexapoda has been 

 acquired secondarily, and that it is a derivative from the progoneatism 

 of Symphyla and Diplopoda, rather than from the opisthogoneatism 

 of Protomyriapoda represented in modern Chilopoda. The establish- 

 ment of the genital openings on the posterior part of the body in the 

 Hexapoda was very probably an adaptation correlated with the con- 

 centration of the locomotor function in the thorax. 



22. — An early specialization among the Protohexapoda gave rise to 

 the modern entognathous Diplura, Protura, and Collembola, small 

 hexapods characterized by a retraction of the mandibles and maxillae 

 into pouches of the head wall closed ventrally by the labium. The 

 identity in the structure of the mouth parts would alone suggest a 

 phylogenetic unity among the above-mentioned groups, but the latter 

 show also a peculiarity in the development of the hypopharyngeal 

 apodemes, which structures, instead of projecting as free arms into 

 the head, as in myriapods and Machilidae, take the form of long 

 internal ridges that, in Diplura and Collembola, diverge posteriorly 

 from the base of the hypopharynx as sclerotic linear inflections of 

 the membranous integument along the folds between the gnathal 

 pouches and the inner surface of the labium. In Protura the two rods 

 are united for a part of their length. These superficial apodemes 

 give attachment to the same muscles as do the internal apodemes of 

 other forms, and in Collembola they support an elaborate "tentorial" 

 superstructure. In many other respects the entognathous hexapods 

 are widely different from one another, and their inter-relationships 

 are by no means clear. Except for the common characters above 

 mentioned, they might be supposed to have had quite separate origins 

 from protosymphylan or protohexapod ancestors (see Imms, 1936, 

 fig. 11). They represent abortive lines of evolution that have not 

 led to higher forms. 



The Diplura depart least from the thysanuran branch that has given 

 rise to the winged insects, since they retain the abdominal styli and 

 cerci, and have the usual hexapod position of the genital openings. 

 The Protura preserve a remnant of the primitive anamorphism of 

 the hexapod ancestors, inasmuch as the last two somites are formed 

 during postembryonic development, but they lack antennae, styli, and 

 cerci ; the small appendicular organs on the first three abdominal 

 segments may be coxal remnants of limbs, with eversible vesicles in 

 10 



