53 



ancestral Arachnida. The extinct Trilobites were possibly 

 derived from this part of the Tracheate series, while the 

 Poecilopoda* {Limidiis) , and their ancestral allies the 

 Eurypterida form a branch probably from this region of the 

 primitive Arachnida. 



From about the same point must have diverged the 

 ancestral series leading upwards to the higher Arachnida. 

 Of these the Arthrogastra {Scorpio) are probably the most 

 nearly in the direct line of development, while the Araneina 

 and the Acarida are two divergent and more modified series. 

 The Acarida are probably somewhat degenerate, and the 

 Araneina are the most highly differentiated group in this 

 branch of the Tracheata. In these higher Arachnids pul- 

 monary sacs to a large extent replace tracheae as respiratory 

 organs. 



At the point where the ancestral Insecta (Hexapoda) 

 diverged from the primitive Arachnida and Pantopoda, the 

 number of segments in the body must have become restricted 

 to about seventeen or eighteen, and were arranged in three 

 groups — four constituting the head, three the thorax, and 

 ten or eleven the abdomen. At first these segments were 

 provided each with a pair of appendages, and the Thysanura 

 and Collembola (constituting together the Aptera of the 

 table) may be regarded as the degenerate representatives of 

 such a stage. They are decidedly the most nearly related of 

 all Insects to the primitive wingless ancestors,! and retain, 

 along with other ancestral characters, rudiments of abdominal 

 appendages (Campodea). 



Above the point where the Aptera diverged, certain 

 important changes must have taken place. The abdominal 



* See Bay Lankester, Limulus an Arachnid, Quart. Journ. Micros. Sc, 

 vol. xxi, Nos. Ixxxiii and Ixxxiv. (1881). 



+ See Lubbook, Monograph on Collembola and Thysanura, Bay Society, 

 1873 ; and Balfour, Corny. Emhryol., vol. i, p. 353. 



