. Text-book of embryology. Embryology. Ill EXTERNAL GILLS 157 the mandibular arch which has lost its respiratory and taken on a supporting function. While external gills occur within three main subdivisions of the Vertebrates, namely Teleostomatous fishes (Crossopterygians—the most archaic of existing Teleostomes), Lung-fishes, and Amphibians, there are two main groups—Elasmobranchs and Amniotes—in which they are conspicuous by their absence. Having regard to the tendency of the organs in question to disappear (as in the cases already alluded to amongst the Amphibia) their absence in a special
. Text-book of embryology. Embryology. Ill EXTERNAL GILLS 157 the mandibular arch which has lost its respiratory and taken on a supporting function. While external gills occur within three main subdivisions of the Vertebrates, namely Teleostomatous fishes (Crossopterygians—the most archaic of existing Teleostomes), Lung-fishes, and Amphibians, there are two main groups—Elasmobranchs and Amniotes—in which they are conspicuous by their absence. Having regard to the tendency of the organs in question to disappear (as in the cases already alluded to amongst the Amphibia) their absence in a special group would not in any case constitute strong evidence that they were never present in the ancestors of that group. As it happens however there is in the two groups mentioned a definite cause which seems quite competent to account for the dis- appearance of external gills, namely the de- velopment of a new organ — the yolk - sac with its highly developed vitelline network of blood-vessels—which in addition to its primitive function must neces- sarily also function as a very efficient organ of respiratory exchange and so render any pre-exist- ing respiratory organ no longer necessary. Taking into consideration the presence of external gills in three archaic groups of Vertebrates it seems to the present writer to be clearly indicated that these organs are a very ancient characteristic of the Vertebrate phylum. The only alternative indeed is to regard them as having become evolved independently in the three groups in which they occur. It is difficult to accept this as in any way probable having regard to the similar morphological relations of the organs in question. It might be suggested that somewhere on the course of a large blood-vessel, such as an aortic arch, would be a most natural place for the development of a new respiratory organ. Such a suggestion however is entirely fallacious for simple physical reasons: for new breathing organs will tend to become evolved
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