CEPHALIC SKELETON OE VEKTEBRATA. 445 



new structure, and to the same extent the cephalic skeleton is not 

 new. If the anterior respiratory portion of the body in Amphioxus 

 corresponds potentially to the head of the Craniota, then the 

 skeletal parts in it must be potentially homologous with a cephalic 

 skeleton. This applies to that region of the notochord, and to 

 the tissue derived from it, which encloses the anterior portion of 

 the central nervous system, as well as to the framework of the 

 respiratory cavity. 



In the Craniota this anterior portion of the body is different 

 on the dorsal, as well as on the ventral, surface from what is behind 

 it ; owing to the changes which take place in its functional import- 

 ance, in consequence of its relations to a large number of other 

 organs, it gets to differ in many special points, owing to the exist- 

 ence of which it can be distinguished as the head ; and it conse- 

 quently becomes of greater importance than the rest of the body. 

 It stands in relation to the entrance of the alimentary canal ; it con- 

 tains the most important of the sensory organs ; and affords a shelter 

 to that portion of the central nervous system which is developed 

 into the brain. These relations are, moreover, the cause of the 

 metamorphosis which affects it. 



In the skeleton of the head we may distinguish — -1) the Skull, 

 and 2) the Branchial skeleton. 



1) The Skull (cranium) is the name given to that part which 

 is a continuation of the dorsal column, and is connected with the 

 axial skeleton. It has many things in common with the former, for 

 it corresponds to a number of vertebral centra and upper arches. 

 This similarity is not only implied in its texture, but also in its 

 structural relations, and in its relations to the central and peripheral 

 nervous system. The chorda dorsalis is continued into the basal 

 portion of the cranium, where it may either remain permanently, or 

 disappear after a short time. With increased development of the 

 higher sensory organs the cranium becomes of more importance. 

 A hinder segment encloses on either side the auditory organ, and 

 may be distinguished as the auditory capsule. Going forward, there 

 is, on either side, a depression which affords shelter for the eyes 

 (orbit), while in the most anterior part there are cavities for the 

 reception of the olfactory organ. The primitive condition of this 

 cranium is cartilaginous ; it forms the "primordial cranium." 



2) With this cartilaginous skull there is connected, at the com- 

 mencement of the alimentary canal, a system of arches, which is 

 likewise primitively cartilaginous ; these branchial arches are, on 

 the whole, similar to the ribs of the vertebral column, but at the 

 same time they are not altogether homodyiiamous with ribs. The 

 various arches differ in form, but they all give indications of having 

 been primitively similar. Their difference in form is due to a 

 differentiation which has been brought about by adaptation to 

 different conditions. 



