?j;6 COMPARATIVE ANATOMY 



in the center of the velum and becomes greatly reduced in size. The asym- 

 metr". ' - ' :he hyjK^rtrv>phy aj>j>ear to be larval adaptations, 



I - 'XT or not gilb are metameric structures has l>een 



an open one. rhe metamerism of chordates b manifestetl primarily in 

 the r V .il s^>mites, Sim-e there are none of these in hemichordates 

 and . ues. it is impcT>ssible to demonstrate in these forms a v-or- 



respondence between mesomerisana and branchiomerism. and thus to 

 establish the metamerism of the latter. The case is different. howe\-er, 

 in amphioxus, where the mesodermal segmentation is one of the most 

 striking features. In the adult animal, there is no correspondence 

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

 take an internvetameric position in relation to the myv>tomes, but also are 



"^ 



aiwJ ,\mTOocoet«*. and (If" - ^ c»->j»h8vsvis; 



«, thyroid gUuad. (.From Kxacsicy s Cocnporanvv Anauwoy ot V«wtorat«s.") 



innervated by metameric nerves, .\ similar metameric correspondence is 

 s ' ' \^stomes. The conclusion drawn 



> corresjxmd. 



rhe number oi gills varies greatly in different c>xlostomes. In the 



' * " - ., the numt>er ranges fK>m fourteen to six pairs. The 



- and petrv>myzon is res^^ev lively si\ and seven, or one 



more counting the spiracular pouch which does iK>t become perforate. 



By the Ivickwan^i grv)wth of the hyoid septum, the external apertures in 



myxine l>ev\>mes reduced to a single pair, a condition not unlike that in 



bony fishes. The posterior displacement of the gills in bdellostoma is an 



' - i>arasitic habit oi this animal, which burrows with its 



: - - of its host. 



Among elasmobranchs, Heptanchus has seven pairs of gill slits in 

 .^ ' ' ^ \s, which are evidently mw' " ' : ' slits since they 



:>ranchs, Hexanchus an*.: .i have six pairs 



of gill shts. Most eiasmobraiKhs ha\-e nve pairs of gili slits plus spiracles, 

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

 absent. 



Gill slits disappear in adult tailless amphibia, but are present in 

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

 urvxleles have three pairs oi gill slits, some two. and some only one. 

 In the newts they disapj>ear entirely. Cutaneous respiration b common 



