The first principles of the present work I laid down in my small pamphlet entitled _Grundriss der Naturphilosophie, der Theorie der Sinne und der darauf gegrundeten Classification der Thiere_; Frankfurt bey Eichenberg, 1802, 8vo (out of print). I still abide by the position there taken, namely, that the Animal Classes are virtually nothing else than a representation of the sense-organs, and that they must be arranged in accordance with them. Thus, strictly speaking, there are only 5 Animal Classes: _Dermatozoa_, or the Invertebrata; _Glossozoa_, or the Fishes, as being those animals in whom a true tongue makes for the first time its appearance; _Rhinozoa_, or the Reptiles, wherein the nose opens for the first time into the mouth and inhales air; _Otozoa_, or the Birds, in which the ear for the first time opens externally; _Ophthalmozoa_, or the Thricozoa, in whom all the organs of sense are present and complete, the eyes being moveable and covered with two palpebræ or lids. But since all vegetative systems are subordinated to the tegument or general sense of feeling, the Dermatozoa divide into just as many or corresponding divisions, which, on account of the quantity of their contents, may be for the sake of convenience also termed classes. Thereby 9 classes of the inferior animals originate, but which, when taken together, have only the worth or value of a single class. So much by way of explaining the apparent want of uniformity in the system. I first advanced the doctrine, that all organic beings originate from and consist of _vesicles_ or _cells_, in my book upon Generation. (_Die Zeugung._ Frankfurt bey Wesche, 1805, 8vo.) These vesicles, when singly detached and regarded in their original process of production, are the infusorial mass, or the protoplasma (_Ur-Schleim_) from whence all larger organisms fashion themselves or are evolved. Their production is therefore nothing else than a regular agglomeration of Infusoria; not of course of species already elaborated or perfect, but of mucous vesicles or points in general, which first form themselves by their union or combination into particular species. This doctrine concerning the primo-constituent parts of the organic mass is now generally admitted or recognised, and I need not, therefore, add anything by way of apology for it or defence.