60 THE ENTOMOLOGIST. 



before was obscure, and have poinled out some striking 

 resemblances, or affinities, as it is the fashion to call them. 

 You have proved to my satisfaction the centrality of certain 

 groups or types of form containing some of the charac- 

 teristics of the surrounding groups, together with a character 

 peculiarly their own. This, it appears to me, must be the 

 key to affinities, if such exist. That there are really seven 

 great and perfectly natural grou])s of insects, and that they 

 approach each other as you have represented, appears 

 undeniable. Divide any one of them, and the parts lose 

 their relative value when compared with the other groups." 

 — Extract from a letter from Dr. Harris to E. Newman^ 

 dated January 1th, 1844 ; and published in the Memoir 

 of Dr. Harris, by Col. T. W. Higyinson, prefixed to the 

 Entojnological Correspondence of T. IV. Harris, edited by 

 Samuel H. Scudder, 1869. 



At the risk of being considered prosy in the repetition of a 

 thrice-told tale, I will repeat Cuvier's " distribution of animals 

 according to their organisation," and define four groups, 

 which, though virtually identical with those I am about to 

 employ, have different names. The divisions are these: — 



1. Endosleate a)timals, having an endo-skeleton, or 

 internal iVamework of bone, to which the muscles are 

 attached; the muhcles clothe and cover the endo-skeleton, 

 and both are enclosed in a sack, called the skin. VVe are 

 told by anatomists that this endo-skeleton is continually 

 undergoing disintegratiouj absorption, and renewal ; but of 

 this I am incapable of forming an opinion, still less can 1 

 describe any portion of the process. Nevertheless, seeing 

 that the exo-skeleton of the next group is repeatedly 

 discarded and reproduced, I am perfectly ready to admit 

 an analogous phenomenon may exist in the endo-skeleton, 

 although the process by which it is performed is so widely 

 different that one fails to follow it in all its details. [These 

 are the Vertebrata of Cuvier.] 



'2. Exosteate animals, which have no internal frame- 

 work of bone, but, in its stead, an indurated skin, enveloping 

 and enclosing the softer parts ; and this 1 call the exo- 

 skeleton, or external skeleton. This answers the same purpose 

 of protection and suj))3ort to the muscles as the endo-skeleton, 

 but its position is exactly the reverse. The exo-skeleton, as 



