128 



Comparative Morphology of Chordates 



Tail-fm heterocercal (Figs. 327, 332A). Spiracles usually open; five 

 pairs of gill-clefts. Air-bladder dorsal to digestive tube and perma- 

 nently open. Spiral valve in intestine well developed. 



These are the sturgeons (Fig. 333 A), found in large rivers and 

 lakes of the northern hemisphere, and the "spoonbill" (Polyodon) of 



r.l. 



—.:.".". :":; " r ,::L_ ' ,„v:,r;v:, , :;:::,;v,v,"v, -:,"■,■. 



ct.c. 



{.-2-I^l. : „ . ffi 



9- 



Fig. 331. Diagram illustrating the air-bladder 

 or lung in fishes, as seen from the left side. (A) 

 Primitive symmetric arrangement. (B) Polyplerus. 

 (C) Neocerafodus. (D) Physostomous teleost. (E) 

 Physoclistic teleost. (a.c.) Alimentary canal; (g.) 

 glottis; (/./.) left lung; (r.l.) right lung. (From Good- 

 rich, "Studies on the Structure and Development of 

 Vertebrates." By permission of The Macmillan 

 Company, publishers.) 



the Mississippi River. A conspicuous feature of the common sturgeon, 

 Acipenser, is the scales, which are massive plates consisting entirely 

 of bone and arranged in rows extending lengthwise of the trunk and 

 tail. The spoonbill is nearly devoid of scales. 



Order Poly pier ini 



Skeleton well ossified, although much cartilage persists in cranial 

 portion of skull. Complete bony vertebrae with centra. Tail-fin 

 diphycercal (Fig. 327i?). Body completely invested by an armor of 

 thick rhomboidal scales (Fig. 328) consisting of bone covered externally 



