8 INTRODUCTION. 



to the eye, it is rational to infer that vestigial organs can- 

 not be the same as rudimentary organs. 



The use of the word "worm" is restricted to members of 

 the subkingdom of Vermes. It is obviously misleading to 

 apply it to such a specialized animal as a young butterfly or 

 moth. 



"Simple" and "rudimentary" are used only in describ- 

 ing primitive and generalized forms or organs. 



"Direct development" applies, as we use it, to the 

 mode of development of the more generalized members of 

 a class. Some naturalists, on the contrary, use it in the 

 sense in which we have used accelerated or abbreviated 

 development; the latter we apply only to the mode of 

 development of the forms specialized by addition or reduc- 

 tion. 



Primitive forms are spoken of as primary, generalized, 

 simple, fundamental ; they have a primitive or a direct 

 development, and some of their organs may exist as rudi- 

 ments, hence the adjective rudimentary. 



Specialized forms, on the other hand, are spoken of as 

 secondary, differentiated, complex, adaptive ; their devel- 

 opment is either indirect (with a metamorphosis), accel- 

 erated (where the early stages are passed through quickly 

 or skipped), or suppressed (where the advanced stages 

 are omitted) ; some of their organs may exist as vestiges, 

 hence the adjective vestigial. 



The invertebrates or animals without a vertebral column 

 are divided primarily into three divisions, the Protozoa, 

 Mesozoa, and Metazoa. The Protozoa are represented in 

 the collection by many forms, the Mesozoa by one species. 

 The Metazoa are subdivided into nine large groups, the 

 Porifera, Coelentera, Echinoderma, Mollusca, Vermes, 

 Crustacea, Arachnozoa, Myriopoda, and Insecta. Some 

 of these groups are again divided, as, for instance, the 

 Insecta, where typical forms of sixteen orders are figured 

 and described. 



