238 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



In the cyclostomes and notidanid sharks the first cleft (between the 

 mandibular and hyoid arches) bears gills like the rest, but elsewhere 

 it differs. In most elasmobranchs and in a few ganoids (Acipenser, 



va 



FIG. 244. Diagram of relations of oesophagus and respiratory tracts in (A) Myxine and 

 Ammocoetes, and (B) Petromyzon, b, bronchus; oe, oesophagus; /, thyreoid gland. 



Polyodon, Polyptems) it becomes reduced in size in the adult, the 

 closure beginning ventrally (fig. 136) so that the persistent part of the 

 opening is on the upper side of the head. This opening is called the 

 spiracle. In other vertebrates, including the 

 chimaeroid sharks and many true sharks, the 

 spiracle is closed in the adult, but in the anura and 

 the amniotes its inner portion persists as the 

 Eustachian tube and the tympanic cavity of the 

 ear (p. 187). 



Usually the series of gills begins with the 

 demibranch on the caudal side of the hyoid arch, 

 while none ever appears on the caudal side of the 

 last cleft. In the teleosts the series of gills is still 

 further reduced, the reduction reaching its ex- 

 treme in Amphipnous, where there are no demi- 

 branchs on the first and fourth branchial arches 

 and only one on the second. 



In the cyclostomes the gill clefts occur at a consider- 

 able distance behind the mouth, partly the result of the 

 great development of the lingual apparatus. In the larvae 

 of Petromyzon (Ammocoetes) the seven gill clefts are 

 nearly typical, the gill extending inward nearly to the 

 pharyngeal wall, each cleft having a short efferent duct 

 leading to the exterior, and the oesophagus beginning at 

 the hinder end of the pharynx (fig. 244, .4). In the meta- 

 morphosis to the adult the oesophagus grows forward, 

 dorsal to the gill clefts, to the cephalic end of the 



pharynx, thus cutting off a ventral respiratory tube, the so-called bronchus (fig. 



244, B). At the same time the gill-bearing region of each cleft becomes separated 



FIG. 245. Gill 

 pouches and blood-vessels 

 of Myxine, after Muller. 

 b, gill pouches, ed, effer- 

 ent ducts; eo, external gill 

 opening; h, heart; oe, 

 cesophageo- cutaneous 

 duct; ph, pharynx. 



