2 28 KINGSLE Y. [Vol. VIII. 



brought out by the present investigations and especially 

 because some of the arguments advanced by the advocates 

 of the arachnidan afifinities of the horse-shoe crab do not 

 seem to be understood by several recent writers. 



Notwithstanding the early suggestion of Strauss-Durckheim 

 {teste Lankester) and the later one by the younger Van 

 Beneden ('7l) there was no serious question of the relation- 

 ship supposed to exist between Limulus and the Crustacea 

 until the publication of Lankester's paper ('8i) " Limulus an 

 Arachnid." Previous to that date there was a general agree- 

 ment that the Arthropods were divisible into two great classes: 

 — Tracheata and Branchiata — the division being based pri- 

 marily upon the method of respiration ; and this view was 

 greatly strengthened by Moseley's discovery ('74) of tracheae 

 in Peripatus, thus apparently providing for a line of descent 

 for the Tracheates without the necessity of any close associa- 

 tion between these and the Crustacea. The Arachnids were 

 of course included in the Tracheata for in most of the group 

 were found tracheae, apparently built upon the same plan as 

 those of the Hexapods, while Leuckart had shown long ago ('49) 

 that the pulmonary sacs of the spiders and scorpions were 

 clearly homologous with the tracheae of the other Arachnids. 



Although not primarily based upon the respiratory system 

 Lankester's conclusions were in substance that the lungs of 

 the Arachnids were homologous with the gills of Limulus ; 

 and the deduction necessarily followed that all Tracheates 

 must have come from a Limuloid ancestor or that the group 

 "Tracheata" must be polyphyletic in origin and that the 

 similarities of the tracheas in Hexapods and Arachnida must 

 be due to homoplassy rather than to community of descent. 



Lankester's paper produced no little discussion and the 

 points presented by the numerous papers upon the subject as 

 well as those based upon the present in'^'estigations may be 

 presented in categorical order as follows :- - 



L Limulus agrees with the Crustacea and differs from the 

 Arachnida in : — 



1. A branchial respiration. 



2. The possession of biramous appendages. 



