ANURIDA. 423 



take place between the tracheae of opposite sides of the 

 body. In Sminthurides and Actaletes Willem states that 

 the tracheal branches are entirely confined to the head, 

 while in Papirius he has discovered two invaginations 

 which correspond in their position with the stigmata of 

 Sminthurus, and regards them as being the last remnants 

 of a tracheal system. 



The heart in the Arthropleona is similar to that of 

 Anurida ; in the Symphypleona it is much contracted, 

 not extending further backwards than about one-third of 

 the length of the abdomen. Willem states that in this 

 sub-order it is only furnished with two pairs of ostia. 



The reproductive system presents very little range of 

 variation ; a median ventral diverticulum of the genital 

 canal may occur in the male (Sminthurus) as well as in 

 the female (Anurida). 



VIL— THE AFFINITIES OF THE COLLEMBOLA. 



The most marked characteristic of the Collembola is 

 the fact that the abdomen consists of only six seg*ments, 

 and at no stage in the development are there known to be 

 more than that number present. Among all other insects, 

 including the Thysanura, wherever the embryology has 

 been studied, the abdomen has been found to consist of 

 not less than ten segments, together with a telson. The 

 significance of this feature of the Collembola has not been 

 emphasised by morphologists, although it is one of the 

 most fundamental characteristics of the order. It is 

 reasonable to believe that this character has been acquired 

 from the primitive ancestors of the Insecta, whose meta- 

 merization had not yet become fixed and limited to a 

 definite number of segments. In this respect, therefore, 

 the Collembola are very much isolated from the rest of the 

 Insecta, and the question arises as to where their affinities 



