2 THE APOD I D^ PART I 



at rest, and to show that Apus must not only be 

 ranked by the side of the Trilobites as one of the 

 primitive Crustacean forms, but that it is itself a true 

 link between the living Crustacea and the Annelida. 

 By careful examination of the organisation of Apus, 

 and a comparison of it with that of a carnivorous 

 Annelid, it is possible to show, as will be done in the 

 following pages, that Apus is perhaps the most perfect 

 " missing link " which zoology so far possesses, perfect, 

 not only because its morphology is easily deducible 

 from that of a carnivorous Annelid, but also because 

 the mechanical causes of the transformation are 

 apparent. The Apodidae will in fact be found to 

 afford us the first complete illustration of the rise of 

 one large animal class out of another by the simple and 

 natural adaptation on the part of one single species 

 of the latter to a new manner of life. Close investiga- 

 tion shows the Apodidse to be both morphologically 

 and biologically an almost ideal transition form. 



More or less satisfactory transition forms be- 

 tween most of the great animal classes are now 

 known, but none has till now been discovered 

 between the Annelida and the Crustacea. The 

 object of this book is to satisfy this want, not by 

 the discovery of a new animal, but by a new ex- 

 planation of one long known and often described. 



The established transition forms between the other 

 classes of the animal kingdom still leave much to 

 be desired. Between the Protozoa and Metazoa 

 the transition forms are either claimed by botanists, 

 or else, however probable, are somewhat hypothetical. 



