GILLS OR BRANCHIiE 



259 



In recent years considerable doubt has been thrown on the entodermal 

 character of the piscine gills. In the first place, the amphibian gills 

 are certainly ectodermal, and it is improbable that structures so 

 similar as the gills of these two groups should have developed in two 

 different ways in the vertebrate phylum. In several fishes there is 

 evidence that the ectoderm grows inward along the line where the 

 cleft is later to open (fig. 271), and this makes it probable that 

 similar conditions exist in all ichthyopsida, although the evidence 

 presented by Petromyzon points in a different direction. The matter 

 is one of great difficulty, and cannot be regarded as settled. 



One of the most prominent differences between the sharks and the skates 

 is in the position of the gill clefts — on the sides of the 'neck' in the former, on 

 the lower surface of the body in the latter — differences brought about by the 

 union of the anterior appendages with the head in the skates. 



Fig. 279. — Breathing valves of teleosts, after Dahlgren. A, schematic figure, the 

 anterior half in the vertical, the posterior in the horizontal plane; B, mouth of sunfish 

 {Eupomotis); b, branchiostegal valve; mn, mx, mandibular and maxillary valves; v, 

 oral valves. 



In the sea horses and pipe fishes (lophobranchs) the gills form small rounded 

 tufts in the gill chamber. In the labyrinthine fishes, many of which can live 

 for some time in the air, there is a complicated bony structure — the labyrinth — 

 in the gill chamber. This is covered by a richly vascular folded membrane 

 which is used in aerial respiration. In some of the symbranchii there is a sac 

 between the hyoid and the first branchial arch which extends back as far as 

 the shoulder girdle and is supplied with venous blood from the branchial vessels, 

 while in the siluroid Saccobranchus a somewhat similar diverticulum from the 

 branchial chamber extends back along side the vertebral column even to the 

 abdominal region. In both the walls are very vascular and the organ is 

 respiratory. 



Many of the teleosts have breathing valves. Of these, the anterior pair 

 are at the mouth opening, attached to the margins of the jaws, and permitting 

 the ingress but not the outflow of water. The second pair is formed by the 



