2094. Through the medium of the bones the distinction between back and belly has been definitely established in the animal, and, as a consequence thereof, the distinction also of right from left. Before a formation of bone exists, the animal is for the most part a round cylinder.
2095. The osseous system can in itself be only symmetrically constructed.
2096. The osseous is the only symmetrical system in an animal. The other organs are so only in so far as they follow the arrangement of the osseous system.
2097. The animal osseous system is, from its being a repetition of the intestinal canal, a tube. This tube is surrounded, like the trachea, by rings, between which the tegumentary tube suffers constrictions.
2098. The back is a series of numerous bony rings.
2099. These bony rings are the bodies of the vertebræ.
2100. The vertebræ have originated through polar repetition, through the muscular cysts.
2101. In addition to the series of vertebræ on the back, a vertebral column is formed moreover along the ventral surface, and without doubt there only where the air-organ, the branchia or lung, is situated. This inferior vertebral column is the breast-bone or sternum.
2102. All the systems, even those that are of a subordinate character in the animal, follow the direction of the main vertebral column. The intestine, as well as the vessels, are placed in accordance with it. Thus the principal trunks of the vessels take their rise along the vertebral column, the other vessels being given off from them in this situation, like the lymphatics from the intestine.
2103. The vascular ramules which surround the intestine and the skin, run out from a main stem, and are directed in a symmetric manner downwards and upwards (in the horizontal body of animals), or towards the belly and back.