£-yQ STRUCTURE OF VERTEBRATES 



which they occupy. The eye occupies segments one, two, and three, 

 the myotomes of which form its muscles, and the ear, with its 

 containing skeletal capsule, occupies segment four so completely 

 that its myotome is obUterated and disappears, and segment 

 five so much that its myotome becomes vestigial. The remaining 

 metotic myotomes contribute to the musculature of the epi- 

 and hypobranchial regions. 



A summary of the segmentation of the head is given in Table 

 Vn. In some gnathostomes the number of metotic segments 

 is different, both because the auditory capsule may obUterate more 

 than two, and because a variable number is added in the occipital 

 region. The primitive number of head segments is perhaps ten, 

 which would correspond to seven pairs of gills, the number 

 found in the shark Heptanchus. In Anura the ' head ' is much 

 shortened, as only five segments remain in the skull. 



We can now consider what should be included in the term 

 ' head '. In the dogfish, and most other fishes, there is no difficulty ; 

 there is a specialised anterior end of the body, containing dorsally 

 the skull and brain and chief sense organs, and ventrally the 

 mouth and gills, and although this is not divided physically 

 from the trunk by any sort of constriction, it has been shown 

 to correspond to a definite number of segments and there is 

 no doubt where its posterior boundary lies. It may be defined 

 shortly as that part of the body which is in front of the shoulder 

 girdle. The importance of this girdle as the dividing line is seen 

 very clearly in the teleosts, where it borders the last gill sUt and 

 is attached dorsally to the skull. In the tetrapods two difficulties 

 occur. The first, that a variable number of segments takes part 

 in the occipital region of the skull, has already been mentioned 

 and need cause little confusion. The number of segments in 

 different regions of the body of vertebrates is obviously highly 

 variable, and no one is likely to try to define sacrum or fore-limbs, 

 for example, in terms of the segments from which they are 

 formed ; similarly we cannot expect to be able to define the head 

 or the skull in this way. The second difficulty is greater. When 

 the gill slits cease to be functional the ventral part of the head 

 loses its importance, and can be suppressed or converted to other 

 uses. In fact, part of the branchial skeleton and its musculature 

 is transformed into the tongue and its supports, and remains 

 within what is usually called the head, while the rest passes 

 backwards so that it comes to lie beneath the anterior vertebrae 



