THE ANIMAL KINGDOM 25 



Dorsal and ventral spinal nerves join to form mixed trunks. Sympathetic 

 ganglia are differentiated. The liver has at least two lobes. 



Of special interest are the orders of fishes which are believed to be in 

 the line of ancestry of land animals. Cartilaginous forms Hke the Elasmo- 

 branchs (sharks and skates) were the common stock from which the remain- 

 ing orders of fishes were probably evolved. 



The extinct Crossopterygians or lobe-finned fishes, which make their 

 first appearance in the Devonian, were air-breathers and possibly the 

 direct ancestors of land animals. Arthrodires are fossil forms possibly 

 related to the modern lung-fishes or Dipnoi. The Dipnoi have either one 

 or two lungs and are in many ways transitional between fishes and 

 amphibia. Ganoids and Teleosts are "ray-finned" forms which include 

 the greater number of living species of fish. 



Class Amphibia 



Amphibians bridge the gap between land and water vertebrates, since 

 some, like fishes, are lungless, and when lungs are present, either permanent 



NECTURUS. 



Fig. 19. — Necluriis, a urodele Amphibian, interests zoologists because more than any 

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



or temporary gills occur. 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 urodeles or tailed forms, the newts and 

 salamanders, annra 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 

 of the latter are protected by fetal membranes, while those of the former 

 are without them. 



Class Reptilia 



Reptiles are horny-scaled vertebrates which breathe by lungs, the 

 embryos of which develop in a liquid-filled sac, the amnion. The skull 



