ii GILLS 45 



left side, both opening by one short tube laterally on the left 

 side, at a variable distance between the eye and the vent. Hence 

 the elegant terms of Amphi-, Medio-, and Laevo-gyrinidae (yvplvos 

 being the Greek for tadpole). 



The external gills lead to a further consideration. Protopterus 

 possesses a vestigial external gill on the shoulder-girdle. Lepi- 

 dosiren has them on the gill-arches, resembling piscine internal 

 gills, and Polypterus has a large biserially fringed external gill (in 

 some cases not disappearing until the fish is adult), which starts 

 from the mandibular arch, at the level of the spiracle or first 

 visceral cleft, and overlaps the operculum externally. The axis 

 of this peculiar organ is possibly based upon the homologues of 

 the spiracular cartilages, which themselves are the branchiostegal 

 rays of the dorsal half of the quadrato-mandibular arch. The 

 branchiostegal rays of the hyoidean arch, at least their material, 

 have given rise to the elaborate opercular apparatus ; and, in con- 

 formity herewith, the hyomandibular itself is not known to carry 

 a gill. Quite possibly the large external gill of Polypterus is not 

 serially homologous with other external gills it may not be a 

 true gill at all, it has perhaps quite a different function- but it 

 seems to throw light upon a mysterious pair of organs which are 

 common in larval and young Urodela, in the larval Aglossa and 

 in the Apoda. These are the " balancers." 



In Triton taeniatus, before hatching, there appears a little 

 protuberance behind and below the eye ; it rests upon the angle 

 of the mandibular arch, and is separated from the first trans- 

 verse, externally visible ridge of the first branchial arch by the 

 beginnings of the hyoidean arch. A few days later the arteria 

 hyomandibularis sends a vessel into this knob, forms a vascular 

 coil, and leaves it as a vein which, instead of returning into the 

 arterial arch, passes into the veins of the body. Its epithelium 

 is not covered with flat, but with cubical cells ; and sensory cells 

 have not been found in it. These organs attain some size, and are 

 shaped like rods, with thickened ends ; they are movable, and are 

 used by the larvae as " balancers," keeping the head from sinking 

 into the slime at the bottom. But they may have other functions 

 besides, and it is not unlikely that they develop into sensory 

 organs like feelers. They occur in many Salamandridae, and are 

 not reduced until, or even after, the metamorphosis, and during this 

 time they shift their place with relation to the eye and the mouth. 



