XIII 



PHYLUM CHORDATA 



235 



filaments are replaced by curious tufted processes (Fig. 881, B, #.). 

 As a rule gills (holobranchs) are developed on the first four 

 branchial arches, but the fourth is frequently reduced to a hemi- 

 branch, 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 Teleostomi the mechanism of respiration is similar to what has 

 already been described in the case of the Trout, and respiratory 



FIG. 802. A, Anabas scandens (Climbing Perch). 

 B, dissection of head, showing accessory respiratory 

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



valves 'are developed in the mouth-cavity. But there are con- 

 siderable differences in details, more especially as regards the 

 relative importance of the opercula and the branch iostegal 

 membranes in carrying on the movements of inspiration and 

 expiration. 



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 Region (Fig. 892) the superior pharyngeal 

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



