THE GILL ARCHES AND THE EXTERNAL GILLS. 261. 



surrounded by at least three distinct pairs of segmentally arranged arches, com- 

 parable on the one hand with the gill arches of the postoral region, and on the 

 other with the cephalic appendages of arthropods. 



The prevalent view, first advanced I believe, by Gegenbaur, that there is in 

 vertebrates but a single pair of oral arches consisting of the mandibles the maxil- 

 lae being regarded merely as a forward extension of their proximal ends was 

 based largely on the relations of the skeletal structures of adult fishes, and is 

 clearly untenable. 



IV. THE GILL ARCHES AND THE EXTERNAL GILLS. 



The oral arches are clearly comparable with the gill arches that lie behind 

 them. In the embryos of primitive vertebrates, however, the gill arches, owing 

 to their more posterior position and to the presence of cardiac elements on their 

 lateral margins, do not unite on the haemal surface of the head, although the more 

 anterior ones move almost as far in that direction as the oral arches do. (Figs. 

 162, 163, 168.) 



A conspicuous feature of the gill arch in primitive vertebrates is the external 

 gill. Budgett has well described them as follows. He states (p. 274), that in 

 Lepidosiren, Protopterus, and in the more primitive amphibia, each gill "arises as 

 an outgrowth from the outer side of the visceral arch, and is composed of a mesen- 

 chymatous core with ectodermal covering. . . . They develop well before the 

 perforation of the gill clefts. . . . and the aortic arch itself is in early stages 

 simply the vessel of the external gill." He concludes that the external gills are 

 organs of great antiquity, which were probably characteristic of primitive verte- 

 brates, not merely larval adaptations of no special significance. 



For my own part, I see no reason to doubt that the external gills of vertebrates 

 represent the remnants of the thoracic appendages of their arthropod ancestors, 

 for they strongly resemble them in form, position, and direction of growth. Com- 

 pare, for example, the external gills of an embryo Protoperus (Fig. 173), with those 

 of an arachnid (Figs. 26-32), and observe also the identity in their arrangement 

 on the mercator projections (Figs. 157, 158), and the details of their structure and 

 mode of branching. 



In many primitive vertebrates, similar organs are found on the oral arches. 

 In the larvae of Amblystoma (Figs. 168, 169), there are long rod-like appendages 

 attached either to the mandibular or to the hyoid arch. These so-called " balan- 

 cers" are clearly serially homologous with the external gills of the more posterior 

 arches. Similar organs are found in other amphibia, as in Zenopus (Fig. 170); 

 the great oar-like appendages of the ostracoderms are in all probability of a 

 similar nature. 



In the frog, the adhesive discs appear to represent the remnants of vestigial 

 appendages or external gills, belonging to either the mandibular, or the hyoid 

 arch. Note for instance their positions in Figs. 161, E and F and in Figs. 164, 



