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, 
Fic. 244.—Diagram of relations of cesophagus and respiratory tracts in (A) Myxine and 
Ammoceetes, and (B) Petromyzon, 6, bronchus; ve, esophagus; #, thyreoid gland. 
Polyodon, Polypterus) 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 
chimeroid 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 
eee great development of the lingual apparatus. In the larve 
FIG. 245.—Gill of Petromyzon (Ammoceetes) the seven gill clefts are 
pouches and blood-vessels nearly typical, the gill extending inward nearly to the 
. rr eal oe ee pharyngeal wall, each cleft having a short efferent duct 
ee eo, external gill leading to the exterior, and the esophagus beginning at 
opening; h, heart; oe, the hinder end of the pharynx (fig. 244, A). In the meta- 
fee liet garip crea morphosis to the adult the cesophagus 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 
