18 Alexander Pclrunkevitch , 



A definite answer to the third question cannot be given at the 

 present state of our knowledge. This much seems to be beyond 

 doubt, that appendages do change their function in pas. ing from 

 a post-oral to a prae-oral position. Thus the two anterior pairs of 

 appendages in the Nauplius of lower crustaceans serve as swimming 

 legs, while the homologous appendages in adult Malakostraca are 

 antenniform sense organs. On the other hand the chelicera of 

 arachnids are still usually functioning as mouth parts although 

 in somewhat different way owing to their position in front of the 

 upper lip. They have, however, lost their gnathobase which is still 

 present in the embryological stages. 



From the foregoing it does not appear reasonable to use the true 

 segmentation of the head as basis for a classification of arthropods 

 before new embryological data are brought in support. The very 

 fact that the segmentation is of primal importance should caution 

 us against drawing conclusions from insufficient premises. The 

 classification which I give further on, is therefore based on characters 

 ascertainable even from palaeontological specimens although derived 

 mainly from the comparative anatomy of recent forms. As the diffe- 

 rent systems proposed by various investigators are to be found 

 in the recent monograph of Pocock, I refer the reader who is desirous 

 of becoming acquainted with them, to this monograph. The dis- 

 cussion of Pocock's system will be found in connection with the 

 definition of each separate order. 



Phylogenetic Development of the Arachnida. 



Little may be said concerning the phylogenetic development of 

 arthropods. That they have developed from chaetopodous seg- 

 mented worms appears to be the only possible conjecture. But the 

 lines along which they developed are so different and the forms 

 so spezialized that the assumption of a polygenetic origin wdth 

 diverging and converging development seems to be more probable 

 than that of a gradual branching of a common ancestral stock. By 

 this I mean that the different classes of arthropods must have de- 

 veloped not from one ancestor, but at different times and from 

 different species of chaetopodous worms, and that this may be true 

 even for some orders of the same class. 



Until quite recently the origin of the scorpions presented, as it 

 seemed, the least difficulties. The segmentation of their body, the 

 shape and size of the cephalothorax, the number of appendages, the 

 presence of a simple telson, remind so strongly of Eurypterids that 

 their derivation from these aquatic arachnids appeared to be much 



