1 6 CHORDATE ANATOMY 



Ganoids are "ray-finned" forms with either cartilaginous or bony 

 skeletons. Their gills are covered with an operculum; they have a spiral 

 valve; the tail is heterocercal; their air-bladder is connected with the 

 pharynx or esophagus by means of an open duct; their skin has ganoid 

 scales, or sometimes, bony scutes, or it may be naked. The order of 

 Crossopterygii, "lobe-finned" ganoids, which make their first appearance 

 in the Devonian period, were air-breathers and possibly the direct ancestors 

 of land animals. The Nile " bichir, " Polypterus, is a Hving representative 

 of this largely extinct group. The sturgeon is a famiUar example of the 

 ganoid group. 



Teleosts are "ray-finned " forms with a wholly bony skeleton. Unlike 

 the ganoids their tail is never heterocercal. They are usually scaly but 

 may be scaleless. The air-bladder when present does not have an open 

 duct; they lack a spiral valve. Teleosts are the most abundant of fishes. 

 The cod and salmon are familiar types. 



Arthrodires are fossil fishes possibly related to the modern lung-fishes 

 or Dipnoi. The Dipnoi are not believed to be the ancestors of land forms, 

 but they are in many ways transitional in structure between fishes and 

 amphibia. They may have either one or two lungs. The Dipnoi are 

 represented by the Ceratodus of Australia. 



Class Amphibia 



Amphibians bridge the gap between land and water vertebrates. 

 Either permanent or temporary gills occur. Lungs are usually present, 



NECTURUS. 



Fig. i8. — Necturus, a urodele amphibian, interests zoologists because more than any- 

 other living amphibian it resembles the fossil Stegocephala, another " ancestral " group. 



but some are lungless. Except in some fossil forms, scales are lacking 

 in the skin. The olfactory pits communicate with the mouth cavity by 

 means of narial passages. The paired appendages are toed. The heart 

 is three-chambered. A postcaval vein is present. The embryo develops 

 without an amnion. 



Amphibia are subdivided into Urodela or tailed forms, the newts and 

 salamanders, Anura or tailless forms, the frogs and toads, and the Gym- 

 nophiona or limbless types. Besides these living orders of amphibia, the 

 fossil order Stegocephala is important, since they appear to be the direct 

 ancestors of reptiles. 



Fishes and Amphibia have been grouped together as Ichthyopsida in 

 contrast with Sauropsida which includes reptiles and birds. The embryos 



