204 THE COCKROACH. 



class, is an attempt too hazardous for a student's text-book.* 

 A review of the facts of Arthropod development led Balfourf 

 to conclude that the whole of the Arthropoda cannot be united 

 in a common phylum. The Tracheata are probably "descended 

 from a terrestrial Annelidan type related to Peripatus. 

 The Crustacea, on the other hand, are clearly descended from a 

 Phyllopod-like ancestor, which can be in no way related to 

 Peripatus." The resemblances between the Arthropoda appear 

 therefore to be traceable to no nearer common ancestors than 

 some unknown Annelid, probably marine, and furnished with a 

 chitinous cuticle, an ccsophageal nervous ring, and perhaps with 

 jointed appendages. Zoological convenience must give place to 

 the results of embryological and historical research, and we 

 shall probably have to reassign the classes hitherto grouped 

 under the easily defined sub-kingdom of Arthropoda. 



Sir John Lubbock has explained, in his very interesting 

 treatise on the Origin and Metamorphoses of Insects, the 

 reasons which lead him to conclude " that Insects generally 

 are descended from ancestors resembling the existing genus 

 Campodea [sub-order Collembola], and that these again have 

 arisen from others belonging to a type represented more or less 

 closely by the existing genus Lindia" [a non-ciliated Rotifer]. 



Present knowledge does not, therefore, justify a more definite 

 statement of the genealogy of Insects than this, that in com- 

 mon with Crustacea they had Annelid ancestors, and that 

 Lindia, Peripatus, and Campodea approximately represent three 

 successive stages of the descent. When we reflect that Cock- 

 roaches themselves reach back to the immeasurablv distant 



IS 



palaeozoic epoch, we get some misty notion of the antiquity and 

 duration of those still remoter ages during which Tracheates, 

 and afterwards Insects, slowly established themselves as new 

 and distinct groups of animals. 



'' Those who care to see a bold experiment of this kind may refer to Haeckel's 

 Schopfungsgeschichte. 



*t* Comp. Embryology, Vol. I., p. 451. 



