XIII 



PHYLUM CHORDATA 



219 



of the interbranchial septa. In the Sturgeon, however, the septa 

 are fairly well developed, reaching half-way up the filaments, so 

 that the latter are free only in their distal portions ; this arrange- 

 ment is obviously intermediate between the Elasmobranch and 

 Teleostean conditions. The most striking deviation from the 

 normal structure occurs in Lophobranchii, in which the gill- 

 filaments are replaced by curious tufted processes (Fig. 832. B y.). 

 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 



FIG. 842. A. Anabas scandens (Climbing Perch). 

 B, dissection of head, showing accessory respiratory 

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



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 rcte mirabile. 



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 (hyo-branchial) gill-cleft. Such sacs are physiologically, 

 though not morphologically, lungs. In the Climbing Perch 

 (Anabas) of the Oriental Region (Fig. 842) the superior pharyngeal 

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



