a. Vegetative Osseous System.
2086. The vegetative osseous system is divisible into dermal, tracheal, intestinal, and vascular bones.
2087. The dermal bones are tegumentary rings, which surround the whole body, and are tracheal rings in so far as the skin is originally a respiratory organ. Such are the rings of the body in Insects, the shells of the Gasteropods and Molluscs, with scales and scutes in general.
2088. The tracheal bones are the branchial arches and tracheal rings.
2089. The intestinal or splanchnic bones are tubes environing the intestine, as in the corals, or imperfect annular segments, which at one time are found in the stomach, as in the Mollusca, at another in the pharynx, as in the Worms, Snails, Sea-urchins, and Holothuriæ; constituting what are called pharyngeal maxillæ. The branchial organs are fundamentally also none other than pharyngeal rings. The lingual and palatal bones, with the intermaxillary bones, belong also to the same category.
2090. The vascular bones are displayed in the hearts of many animals. The three last divisions may be called visceral or splanchnic bones; and then we have tegumentary, splanchnic, and nerve-bones.
b. Animal Osseous System.
2091. The animal or nervo-osseous system must separate from the vegetative system of bones, and be placed upon the side exposed to the light. The side of the inferior animal that is exposed to the light, or averted from the earth, is the upper surface, dorsal region or back.
2092. The back holds the same relation to the ventral side as light does to the darkness, as sun to the earth; therefore the dorsal side is of a dark, the ventral of a faint or pale colour.