THE HEAD PROBLEM 



627 



There appear to be four or five prootic metameres. The number of 

 metaotic segments varies in different vertebrates, depending on the 

 number of occipital vertebrae added. In elasmobranchs five or six have 

 been counted. From this evidence of metamerism in both head and 

 trunk region of vertebrates, most morphologists conclude that verte- 

 brates have evolved from metameric ancestors. Such a metameric 



TRIG. 



Ill / iv 



GLOSSOPHARYNGEAL _ 



PROF 



V, 



N.TERM. V GILL SLITS 6-8 



AORTIC ARCHES 



Fig. 519. — A diagram of the primitive segmentation of the vertebrate head, based 

 chiefly upon elasmobranch embryos. The majority of morphologists are skeptical of 

 the assumption that there ever was a gill-slit between the mouth and the spiracle or 

 first gill-slit.. Possibly amphioxus embryos present such evidence. Neuromeres 

 numbered by Roman numerals, somites stippled, gill-pouches cross-hatched. The 

 cranial nerves have two kinds of ganglion cells — dorsal and epibranchial as shown in the 

 diagram. 



ancestor is best represented today by amphioxus. But, like vertebrates, 

 amphioxus is a chordate. Were the ancestors of chordates metameric 

 animals? Does the metamerism of amphioxus warrant the inference that 

 chordates have evolved from metameric ancestors like the annelids? Or 

 is the metamerism of chordates and annelids a convergent character? 

 The discussion of the head problem thus leads to that of the ancestry of 

 vertebrates. 



