20 - HIGH SCHOOL ZOOLOGY. 



1 i. It is in the vertebral column that the segmentation or 

 metamery of the Vertebrate body finds its most evident ex- 

 pression, but we shall find that many other organs are likewise 

 divided into metameres, notably the muscular and nervous 

 systems. 



15. A study of the development of the catfish shows that not 

 only is the vertebral column built around the notochord, § 1'2, but 

 also that the same is true of a considerable part of the skull, 

 and that the notochord in the head is related in the same way 

 to the nervous system and alimentary canal as it is in the 

 trunk. It was at one time thought that it should be possible 

 to distinguish constituent vertebrae in the skull, but this is 

 impossible, for the very different functions which the anterior 

 part of the axial skeleton has to discharge are associated with 

 corresponding differences in form. Thus the fact that the 

 anterior end of the central nervous system is dilated into the 

 brain, is associated with the development of a sheltering box, 

 the cranium, which is further modified by the apposition or 

 incorporation with it of the protecting hard parts of the higher 

 sense-organs, and the fact that the anterior end of the alimentary 

 canal is devoted to securing food and to respiration is associated 

 with the development of certain hard parts in connection there- 

 with — the visceral skeleton- Of these we shall study first — 



16. The Cranium. — The cranial box has certain openings, 

 one (the occipital foramen or foramen magnum) to permit of 

 the spinal cord joining the brain, others to allow the escape 

 of nerves and the entry or egress of blood vessels. Although 

 originally in the young fish largely cartilaginous in its texture, 

 the box afterwards becomes partly converted into bone, and 

 the bones formed in the cartilage are related in a definite way 

 to these openings. Other bones are formed in the skin for the 

 protection of the sensory canals ; still others (especially in the 

 roof of the mouth) for the support of the teeth, and both of 



