. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. FIG. 244.—Diagram of relations of oesophagus and respiratory tracts in (A) Myxine and Ammocoetes, and (B) Petromyzon, b, bronchus; oe, oesophagus; t, 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 chimaeroid sharks and many true sharks, the spiracle is closed in the adult, but in the anura
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. FIG. 244.—Diagram of relations of oesophagus and respiratory tracts in (A) Myxine and Ammocoetes, and (B) Petromyzon, b, bronchus; oe, oesophagus; t, 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 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 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. FIG. 2*45. — Gill pouches and blood-vessels of Myxine, after Miiller. b, gill pouches; ed, effer- ent ducts; eo, external gill opening; h, heart; oe, cesophageo- cutaneous duct; ph, phary
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1912
