228 



ZOOLOGY 



SECT. 



and further reduction takes place in some cases. The pseudobranch 

 or vestigial hyoidean gill may either retain the characteristic 

 comb-like structure, as in the Trout, or may be reduced, as in the 

 Cod, to a gland-like organ formed of a plexus of blood-vessels and 

 called a vaso-ganglion or rete mirabile. In most Teleostonii the 

 mechanism of respiration is similar to what has already been de- 

 scribed in the case of the Trout, and respiratory valves are developed 

 in the mouth-cavity. But there are considerable differences in 

 details, more especially as regards the relative importance of the 

 opercula and the branchiostegal membranes in carrying on the 

 movements of inspiration and expiration. 



FIG. 903. A, Anabas scandens (Climbing Perch). 

 B, dissection of head, showing accessory respiratory 

 organ. (A, after Cuvier ; B, after Giinther.) 



In addition to the gills some Teleostei possess accessory organs 

 of respiration. In Amphipnous, an Indian Physostome. the gills 

 are poorly developed and are functionally replaced by a vascular 

 sac occurring on each side of the body and opening in front into 

 the first (hyobranchial) gill-cleft. Such sacs are physiologically, 

 though not morphologically, lungs. In the Climbing Perch 

 (Anabas) of the Oriental Kegion (Fig. 903) the superior pharyngeal 

 bones are developed into folded plates (B) covered with vascular 

 mucous membrane and capable of retaining water for a considerable 

 period : the Fish is able to traverse the land, and is even said to 

 climb trees, holding on alternately by the spines of its pre-operculum 

 and of its ventral fins. It has become so thoroughly a land-animal 



