36 STRUCTURE OF THE VERTEBRATES 



quite, absent. This degeneration of the scales often accompanies 

 a loss of fins, and is correlated with mud-living habits. 



4. Gills. The typical fish has internal gills located in the gill 

 slits, serving a respiratory function. In two groups of fish, 

 including the order which is considered ancestral to the higher 

 vertebrates, the larvae have external gills similar to those of 

 the amphibia. 



5. Vascular System. The heart, a twisted, S-shaped structure, 

 has one auricle and one ventricle. The circulatory system is 

 built around the gills, essentially like that of Amphioxus, 

 although with many modifications, 



6. Nervous System. The brain is very simple in character. The 

 cerebral hemispheres are functionless, but the cerebellum is 

 well developed. The eyes and organs of balance are typical of 

 vertebrates from fish up. The latter structure is the only "ear" 

 of the fish, with the result that none of these animals hear as 

 it is understood by man. 



7. Skeleton. The primitive fish have only a cartilage skeleton, 

 but in the higher fish this is replaced by bone. The vertebrae 

 are advanced over the cyclostome condition in having the sepa- 

 rate cartilages fused to form a solid vertebra surrounding the 

 endodermal notochord. In all fish the notochord is constricted 

 by the vertebrae, and in the higher forms is entirely obliterated 

 in the adult. 



The class is divided into three Sub-classes. 



I. The Elasmobranchii, or "plate gilled" fish, which includes 

 the sharks, skates and rays, and are the most primitive fish 

 still existing. 



II. The Teleostomi, or fish with a complete, terminal mouth. 

 It includes practically all the food and game fish — all the fish 

 known to most students, with the exception of the elasmobranchs. 



III. The Dipnoi or lung fish. The name, "double breathing", 

 refers to the fact that they have gills and a lung-like swim 

 bladder. At present there are only three genera in the tropical 

 regions of three continents. 



Elasmobranchii. The elasmobranchs are characterized by a 

 skeleton made entirely of cartilage; and although the group 



