RELATIONS OF INSECTS TO OTHER ARTHROPODA 17 



abdomen well formed, these being, however, before hatching, partly 

 atrophied, so that the body of insects after birth tends to become 

 shortened or condensed. This indicates the descent of insects from 

 ancestors with elongated polypodous hind-bodies like Scolopendrella. 

 Korschelt and Heider suggest that the stem-form of myriopods was 

 a homonomously jointed form like Peripatus, consisting of a rather 

 large number of segments, but we might, with Haase, consider that 

 the great number of segments which we now find indicates a late 

 acquisition of this form. 



The genital opening in chilopods is single, and situated in the 

 penultimate segment of the body, as in insects. While recognizing 

 the close relationship of the Myriopoda with the insects, it still 

 seems advisable not to unite them into a single group (as Oudemans, 

 Lang, and others would do), but to regard them as forming an 

 equivalent class. On the other hand, when we take into account 

 the form and structure of the head, antennae, and especially the 

 shape of the first pair of mouth-appendages, being at least two- 

 jointed in both groups, we think these characters, with the homon- 

 omously segmented body behind the head, outweigh the difference 

 in the position of the genital outlet, important as that may seem. 

 It should also be taken into account that while insects are derived 

 from polypodous ancestors, no one supposes, with the exception of 

 one or two authors, that these ancestors are the Myriopoda, the latter 

 having evidently descended from a six-legged ancestor, quite differ- 

 ent from that of the Campodea ancestor of insects. 1 



In regard to the sexual openings of worms, though their position 

 is in general in the anterior part of the body, it is still very variable, 

 though, in general, paired. In the oligochete worms the genital zone, 

 with the external openings, is formed by the segments lying be- 

 tween the 9th and 14th rings, though in some the genital organs are 

 situated still nearer the head. The myriopods, which evolved from 

 the worms earlier than insects, appear to have in their most primi- 

 tive forms (the Diplopoda) retained this vermian position of the 

 genital outlets. In the later forms, the chilopods, the genital open- 

 ings have been carried back to near the end of the body, as in 

 insects. From observations made by three different observers on 

 the freshly hatched larva of the Julidfe, it appears that the an- 

 cestral diplopods were six-footed, or oligopod, the larva of Pauropus 



1 The term which we proposed for this hypothetical ancestor of insects, " Leptus- 

 like " or " Leptiform," was an unfortunate one, since the name Leptus was originally 

 given to the six-legged larva of a mite (Trombidinm), the origin of the mites and 

 other Arachnida being entirely different from that of the myriupods and insects, 

 c 



