IV PREFACE. 



As to the Artliropoda, its right to exist as a group coordi- 

 nate with, for instance, the Mollusca has been questioned by 

 several authors. Undoubtedly in this case also many similar 

 structural features obtain among the various members of the 

 group, but embryology has indicated a probability of a more 

 or less independent origin of two Arthropodan groups usually 

 regarded as closely related, namely, the Arachnida and the 

 Tracheata proper. Apparently the former have originated 

 from Crustacean ancestors, while, if the supposed significance 

 of Peripatus be accepted, the Tracheates are to be traced back 

 to Annelidan forebears, and for the purpose of calling the 

 attention of the student to this probable phylogeny the 

 Crustacea, Arachnida and Tracheata have been regarded as 

 distinct types coordinate with the Annelida and Mollusca. 



A book of this kind must necessarily be highly tinged with 

 the individual opinions of the writer, and for these indulgence 

 must be craved. So far as the facts are concerned every 

 care has been taken that they should be accurate and as 

 far as possible up to date with the most recent investigations. 

 Errors have no doubt crept in, a misfortune almost inevitable 

 for the mass of material which must pass under consideration 

 during the progress of the work, and for these again indul- 

 gence must be asked. 



Refraining from further apologies, the more pleasant duty 

 remains of thanking the many friends who have so kindly 

 aided the work by suggestion or otherwise, and especially 

 those who have permitted the use of figures taken from 

 special papers. A large number of the figures employed 

 are original and the great majority have been especially 

 drawn for this work, the attempt being made to diagramma- 

 tize them to a greater or less extent for the sake of clearness. 

 In all cases where figures have been borrowed the original 

 authorship has been duly acknowledged. 



Finally, I desire to make public recognition of my indebt- 

 edness to my wife for the invaluable assistance she has ren- 

 dered in many ways during the progress of the work. 



J. PLAYFAIE McMuEKicn. 



UNIVERSITY OP MICHIGAN, 

 September, 1894. 



