THE STRUCTURE OF THE VERTEBRATES 329 



2. The bones consist of the vertebra, the skull, and The 

 the pectoral and pelvic girdles with their appendages, column* 1 

 the limb bones. To the vertebrae are directly articu- 

 lated the ribs, which in the higher groups join the breast 



bone or sternum on the ventral side. In fishes and 

 snakes there is no sternum. A few other bones occur- 

 ring in various animals are not directly articulated to the 

 main skeleton. The replacement of the dorsal elastic 

 rod by bone necessitated the formation of separate 

 pieces or vertebrae ; otherwise the then aquatic animal 

 would have been unable to swim with any success. On 

 the functional side the case is parallel to that of the 

 arthropods, which developed separate rings in their 

 hard chitinous exoskeleton as seen, for example, in 

 the centipedes. A typical vertebra consists of a cen- 

 trum or main body, from which arises above the neural 

 arch, inclosing the neural canal, containing the spinal 

 cord. The spinal cord is developed in the embryo 

 round the primitive groove (the central canal which it 

 contains is a relic of this), and thus belongs to the ecto- 

 derm or outer tissue. The vertebral column, on the 

 other hand, has quite a different origin, from the meso- 

 derm or middle tissue, but in the course of development 

 it surrounds and incloses the cord. In many fishes the 

 notochord remains between the vertebral centra, which 

 may then be deeply excavated in front and behind ; 

 such vertebrae are called amphiccelous. In addition to 

 the characters mentioned, vertebrae frequently exhibit 

 well-marked dorsal spines and transverse or lateral 

 processes. 



3. The skull consists of a number of bones, which are The skull 

 for the most part firmly articulated together. The 

 mandible or lower jaw, which is movable, is not primi- 

 tively part of the skull at all, but is derived from the 



