52 



and lined by a delicate chitinous cuticle. The appendages at 

 the front of the body were beginning to be specialised in 

 connection with the head and mouth opening, the first pair 

 becoming elongated as sensory antennae, and the three 

 following pairs being modified into jaws. 



It is such an ancestral form as this (Proto-Tracheata) 

 that Peripatus represents, and that was continued upwards 

 to the primitive Myriapoda. Above the ancestral stage 

 perpetuated by Peripatus the nephridia were lost, and the 

 segmentation of the appendages and the formation of the 

 tracheae became more perfect. The Myriapoda have remained 

 in very much this condition, the body being still formed of 

 a large number of segments showing very little heteronomy. 



From the ancestral Myriapoda the line leading to higher 

 Tracheata diverged and may be traced upwards to the base 

 of the series of Arachnida and Insecta. Near to the 

 ancestral Arachnida may be placed a few aberrant groups 

 with somewhat doubtful affinities. Of these the most 

 distinct is the Pantopoda (Pycnogonida), a class* which is 

 not closely allied to any of the neighbouring groups, and is 

 best regarded as forming a branch by itself which has 

 diverged from the line leading upwards from the Proto- 

 Tracheata and Myriapoda. In the Pantopoda the number of 

 segments in the body has become greatly reduced, the 

 posterior ones (those forming the abdomen in higher 

 Tracheata) being in a rudimentary condition. The append- 

 ages have become enormously elongated, and those around 

 the mouth considerably modified. The tracheae have been 

 altogether lost. 



The Tardigrada and the Pentastomida are more closely 

 allied to the lower Arachnids than to the Insects, and 

 were probably divergent and degenerate off- shoots from the 



• See Hoek, Challenger Zoological Reports, vol. iii, part x, " The 

 Pycnogonida," p. 145. 



