As compared with that of the Turtle or Crocodile, the endoskeleton of the Duck is characterised by: 1. The great lightness of the bones, many of which contain air cavities. 2. The tendency to become ankylosed together shown by many of the bones. 3. The modification of the anterior limbs and girdle for the purpose of flight. 1. THE AXIAL SKELETON. This, as in other vertebrates, is divisible into-- A. The vertebral column. B. The skull. C. The ribs and sternum. A. THE VERTEBRAL COLUMN. The vertebral column of the duck, like that of the great majority of birds, presents a number of well-marked characteristics, contrasting strongly with those of the generality of higher vertebrates. The centra are always without epiphyses. The neck is exceedingly long, about as long as all the rest of the vertebral column put together, and is remarkable for its flexibility. The trunk portion of the vertebral column on the other hand is characterised by extreme rigidity, and the marked tendency shown by the component vertebrae to fuse together into one almost continuous mass. The most rigid part of the vertebral column is that to which the pelvis is united, as no less than seventeen vertebrae take part in the union. The tail of the duck, like that of all living birds, is very short, and the posterior caudal vertebrae are united together, forming the =pygostyle=. The vertebral column may be divided into cervical, thoracic, lumbar, sacral, and caudal regions, but the boundaries between the several regions are ill-defined. THE CERVICAL VERTEBRAE. All the vertebrae anterior to the first one that bears a rib meeting the sternum are regarded as cervical vertebrae. There are therefore sixteen cervical vertebrae, the last two of which bear well developed ribs. All are freely movable on one another.