xii PHYLUM CHORDATA 337 



Craniates it never passes beyond this stage, but remains of 

 the nature of a persistent notochord, as it is termed. But in 

 the great majority the notochord becomes enclosed in a 

 sheath of cartilage, and thus becomes divided up into a 

 number of segments. Eventually ossification sets in, and 

 the series of completely formed bony vertebrae become^ 

 developed. 



As already mentioned, the spinal column does not extend 

 into the head region. The skeleton of this region is the 

 complex cartilaginous or bony structure known as the skull. 

 The chief part of this is a case, the cranium, in the interior 

 of which the brain is lodged, and the walls of which afford 

 support to three pairs of organs of special sense, the nasal 

 or olfactory organs in front, the eyes in the middle, and the 

 ears or auditory organs behind. The cavity of the cranium 

 opens behind by a rounded foramen, the foramen magnum, 

 into the anterior end of the neural canal enclosed by the 

 neural arches of the vertebrae ; and the posterior region of 

 the cranium articulates movably with the first vertebrae of 

 the spinal column. In addition to the cranium the skull 

 or skeleton of the head comprises certain elements known 

 as the visceral arches. The foremost of these forms the 

 jaws, the second is the hyoid, and mainly supports the 

 tongue, the remainder are the branchial arches. 



In the dogfish the cranium remains in the primitive 

 condition of a cartilaginous case, with complete walls and 

 floor, but with the roof partly formed of fibrous membrane. 

 In the lizard and rabbit the substance of the cartilage is 

 replaced by a number of cartilage bones, i.e, bones which 

 take the place of pre-existing cartilage, to which are super- 

 added a number of membrane bones, i.e., bones, the site of 

 which was not preoccupied by cartilage ; the whole united 

 together so as to form a structure of considerable com- 

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