Highest in this type are the Insects, and among these I include Spiders and Centipedes as well as Winged Insects. It is true that the Centipedes have a long uniform body like Worms, and the Spiders have the body divided into two regions like the Crustacea, while the body in true Insects has three distinct regions, head, chest, and hind body; but notwithstanding this difference, both the former share in the peculiar class-character that places them with the Winged Insects in a separate group, distinct from all the other Articulates. We have seen that in the Worms the respiratory organs are mere vesicles, while in the Crustacea they are more highly organized gills; but in Centipedes, Spiders, and Winged Insects, the breathing-apparatus is aerial, consisting of air-holes on the sides of the body, connected with a system of tubes and vessels extending into the body and admitting air to all parts of it. In the Winged Insects this system is very elaborate, filling the body with air to such a degree as to render it exceedingly light and adapted to easy and rapid flight. The general arrangement of parts is the same in this class as in the two others, the typical character being alike in all. We come now to the highest branch of the Animal Kingdom, that to which we ourselves belong,--the Vertebrates. This type is usually divided into four classes, Fishes, Reptiles, Birds, and Mammalia; and though many naturalists believe that it includes more, and I am myself of that opinion, I shall allude here only to the four generally admitted classes, as they are sufficient for my present purpose, and will serve to show the characters upon which classes are based. In a former paper I have explained in general terms the plan of structure of this type,--a backbone, with a bony arch above and a bony arch below, forming two cavities that contain all the systems of organs, the whole being surrounded by the flesh and skin. Now whether a body so constructed lie prone in the water, like a Fish,--or be lifted on imperfect legs, like a Reptile,--or be balanced on two legs, while the front limbs become wings, as in Birds,--or be raised upon four strong limbs terminating in paws or feet, as in Quadrupeds,--or stand upright with head erect, while the limbs consist of a pair of arms and a pair of legs, as in Man,--does not in the least affect that structural conception under which they are all included. Every Vertebrate has a backbone; every Vertebrate has a bony arch above that backbone and a bony arch below it, forming two cavities,--no matter whether these arches be of hard bone, or of cartilage, or even of a softer substance; every Vertebrate has the brain, the spinal marrow or spinal cord, and the organs of the senses in the upper cavity, and the organs of digestion, respiration, circulation, and reproduction in the lower one; every Vertebrate has four locomotive appendages built of the same bones and bearing the same relation to the