252 CHORDATE ANATOMY 



between gills and myotomes. But in the larva, the gill-slits not only 

 take an intermetameric position in relation to the myotomes, but also are 

 innervated by metameric nerves. A similar metameric correspondence is 

 strikingly shown in the embryos of cyclostomes. The conclusion drawn 

 is that mesomerism and branchiomerism correspond. 



The number of gills varies greatly in different cyclostomes. In the 

 genus Bdellostoma, the number ranges from fourteen to six pairs. The 

 number in Myxine and Petromyzon is respectively six and seven, or one 

 more counting the spiracular pouch which does not become perforate. 

 By the backward growth of the hyoid septum, the external apertures in 

 Myxine becomes reduced to a single pair, a condition not unlike that in 

 bony fishes. 



Among elasmobranchs, Heptanchus has seven pairs of gill-slits in 

 addition to the spiracles, which are evidently modified gill-slits since they 

 bear rudimentary hemibranchs. Hexanchus and PHotrema have six pairs 

 of gill-slits. Most elasmobranchs have five pairs of gill-slits plus spiracles. 

 In bony fishes the number is reduced to four pairs and the spiracle is 

 absent. 



Gill-slits disappear in adult tailless amphibians, but are present in some 

 aquatic urodeles. The number however is reduced. Some adult urodeles 

 have three pairs of gill-slits, some two, and some only one. In the newts 

 they disappear entirely. Cutaneous respiration is common in the group, 

 and some respire by means of a highly vascular pharynx. Nevertheless, 

 even in those adult forms which are devoid of functional gills, gill pouches 

 occur in the embryo, and the embryos of Gymnophiona may have as many 

 as six such pouches, suggesting a corresponding number of functional gills 

 in their ancestors. Most amphibian larvae have functional gills. 



Functional gills are lacking in amniotes, but rudiments of gills are 

 represented by transient embryonic pharyngeal pouches and their inter- 

 mediate visceral arches. In the embryos of reptiles, some of the gill-slits 

 usually become perforate and later close. The perforation of gill-slits in 

 mammals is abnormal. Pharyngeal pouches are, however, always formed 

 in the human embryo, and when these become perforate fistulae in the 

 throat, they may persist and require surgical treatment. The presence 

 of five pharyngeal pouches and six visceral arches alternating with them 

 in the human embryo receives its most reasonable interpretation in the 

 evolution theory. 



As has already been explained, the disappearance of the visceral arches 

 in man and mammals is incomplete. The skeletal elements are converted 

 into ear bones, attachment for the tongue, and support of the larynx. 

 Three of the aortic arches also persist, as will be shown in the next chapter. 

 Moreover, in addition to these rudiments there are certain derivatives 

 of the gill pouches which require special discussion. 



