X PREFACE. 



I have especially devoted myself to the collecting and collating as 

 completely as practicable, the numerous new and important facts in 

 the organization of the invertebrate animals, which have as yet been 

 developed. And as occasion presented, I have verified with my own 

 eyes the particular results ; and Avhen I have been obliged to refer 

 to the discoveries and observations of others, I have cited exactly their 

 works. 



I could not exclude Embryology and Histology from this work, for, 

 in these branches, often lies our only means not only to ascertain the 

 true nature of many larval forms among the lower animals, but also 

 to arrive at the correct interpretation of many organs which, in form, 

 position, and arrangements, have no analogues among the higher 

 animal forms. It is only by the aid of Histology that we are able to 

 show that this or that organ is a branchia, a liver, a kidney, an 

 ovary, or a -testicle ; while, in the Vertebrata, which are organized 

 after a few principal types, the signification of most of the organs 

 can usually be easily determined by their position and connection. 



In order to avoid long descriptions, I have, when practicable, re- 

 ferred to plates and figures ; but in so doing I have always endeavored 

 to cite the good and original representations, for I am convinced that 

 many figures wliich are transferred from one book to another, become, 

 at last, so changed as to be quite dissimilar to the original. 



The elaboration of this work having been commenced in 1845, but 

 its completion having been delayed by my change of residence from 

 Erlangen to Freiburg, and partly by a pretty long sojourn of mine on 

 the Adriatic Sea, I have been unable to use the important works 

 which have been published during the last few years, except in the 

 form of a Supplement [additional notes] which will serve to com- 

 plete, to confirm, or to rectify what has been advanced in the body 

 of' the work. 



I take this opportunity to publicly express my gratitude to A. 

 Kdlliker, H. Koch, A. KroJm, C. Vogt, -and H. Stannius, for the 

 friendly and important aid they have rendered me in the completion 

 of this difficult task — not only by the transmission to me of inter- 

 esting and rare marine animals, but also in the communication of 

 important manuscripts and letters, the contents of which they have 

 allowed me to freely use for my work. 



FREiBURa (in Brbisgau), Feb. 27, 1848. 



C. Th. v. SIEBOLD. 



