Fig. 225.—Transverse section through the Bony Framework of a Typical Vertebrate Animal
1. Spinous process of the vertebra. 2. Neural arch. 3. Transverse process. 5. Body of the vertebra. 6. Breast-bone. 7. Rib. The space between 2 and 5 is the neural cavity; and that between 5 and 6 is the visceral cavity
This column is usually composed of a number of elements called vertebræ, each of which gives off two processes that unite and form an arch on its dorsal side, while all the arches form a tube through which passes the central portion of the nervous system.
Below, or on the ventral side of the column, is the body-cavity containing the organs of digestion and circulation; so that if we make a transverse section of a vertebrate animal we find that there are two distinct tubes or cavities—a neural or cerebro-spinal cavity on the dorsal side of the vertebral column, formed by extensions from the substance of the latter, and enclosing the chief portion of the nervous system; and a body-cavity on the ventral side containing the viscera or internal organs.
The above features are highly important, and will always prove quite sufficient to enable us to decide whether any particular animal is a vertebrate or an invertebrate, for it will be remembered that the body of the latter has only one cavity, containing the nervous system as well as the viscera, and that the nervous system is commonly placed along the ventral side, but never along the dorsal. In addition to this the vertebrates never have more than two pairs of limbs, and these are always directed from the nervous system; and the jaws, which are appendages that move in the horizontal plane in invertebrates, are, in the higher animals, portions of the framework of the head and move vertically. In vertebrates, too, there is always a complete blood system, consisting of a heart with two, three, or four cavities, a system of arteries to convey the blood to the different parts of the body, veins to return the blood to the heart, and networks of fine capillaries connecting the former with the latter.
All vertebrates, at an early stage of their existence, have a cartilaginous rod running through the dorsal portion of the body, called the notocord. In some of the lowest animals of the division this rod persists without any important alterations in structure, while in the higher forms it gives place to the series of cartilaginous or bony elements above referred to as the vertebræ; and the arrangement of the vertebrates into their relative positions in the scale of life is based largely on the degree of development of the vertebral column from the notocord. Another interesting feature in the development of a vertebrate is the formation of five or more transverse, archlike thickenings on each side of the digestive tube, just behind the head; and, in the spaces between them, of a series of slits forming a communication between the pharynx and the exterior. These arches and clefts have but a brief existence in many vertebrates, while in others they persist throughout life; and, like other points referred to, they assist us in recognising the relations of the vertebrates to one another.
The vertebrates are divided into the following classes:—
1. Cyclostomata—Lampreys.
2. Pisces—Fishes.
3. Amphibia—Frogs, Toads, Newts, &c.
4. Reptilia—Snakes, Lizards, Tortoises, &c.
5. Aves—Birds.
6. Mammalia—Mammals.