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 when 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 which 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 miné 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. 
Kolliker, H. Koch, A. Krohn, 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. 
Frerpure (IN Breiseau), Feb. 27, 1848. 
C. Tu. v. SIEBOLD. 
