organs of digestion, circulation, respiration, and reproduction; and that higher system of organs, the brain, spinal marrow, and nerves, with the organs of sense, on which all the manifestations of the intelligent faculties depend, and by which his relations to the external world are established and controlled:  the whole being surrounded by flesh, muscles, and skin.  On account of this fleshy envelope of the hard parts in all the higher animals, Oken divided the Animal Kingdom into two groups, the Vertebrates and Invertebrates, or, as he called them, the “Eingeweide und Fleisch Thiere”—­which we may translate as the Intestinal Animals, or those that represent the intestinal systems of organs, and the Flesh Animals, or those that combine all the systems of organs under one envelope of flesh.  Let us examine a little more closely this singular theory, by which each branch of the Invertebrates becomes, as it were, the exponent of a special system of organs, while the Vertebrates, with man at their head, include all these systems.

According to Oken, the Radiates, the lowest type of the Animal Kingdom, embody digestion.  They all represent a stomach, whether it is the simple sac of the Polyps, or the cavity of the Acalephs, with its radiating tubes traversing the gelatinous mass of the body, or the cavity and tubes of the Echinoderms, inclosed within walls of their own.

The Mollusks represent circulation; and his division of this type into classes, according to what he considers the higher or lower organization of the heart, agrees with the ordinary division into Acephala, Gasteropoda, and Cephalopoda.

The Articulates are the respiratory animals in this classification:  they represent respiration.  The Worms, breathing, as he asserts, through the whole surface of the skin, without special breathing organs, are the lowest; the Crustacea, with gills, or aquatic breathing organs, come next; and he places the Insects highest, with their branching tracheae, admitting air to all parts of the body.  The Vertebrates, or Flesh Animals, with their four classes, represent the Bones, the Muscles, the Nerves, and the Organs of Sense.

This theory, according to which there are as many great divisions as there are structural systems or combinations of systems in the Animal Kingdom, seemed natural and significant, and there was something attractive in the idea that man represents, as it were, the synthetic combination of all these different systems.  Oken also, in his exposition of his mode of classification, showed an insight into the structure and relations of animals that commended it to the interest of all students of Nature, and entitles him to their everlasting gratitude.  Nevertheless, his theory fails, when it is compared with facts.  For instance, there are many Worms that have no respiration through the skin, while his appreciation of the whole class is founded on that feature; and in his type representing