PISCES 



225 



9. The nostrils are paired; there are no posterior nares, so the organs 



are exclusively olfactory. 



10. There are no tympanic cavities or ear drums. 



11. The heart is two chambered with venous blood except in the 



Dipnoi where it begins to be three chambered and receives 

 pure blood from the lung as well as impure blood from the 

 body. 



Apart from the Dipnoi, the heart has one auricle, receiving 

 impure blood from the body; one ventricle which drives it 

 through the ventral aorta to the gills, whence the purified 

 blood flows to the head and by the dorsal aorta to the body. 

 There is a sinus venosus receiving the impure blood and 

 sending it into the auricle and thence to the ventricle. 

 The conus or bulbus arteriosus, located at the exit of the 

 arterial trunk from the heart, becomes more bulbous in the 

 Dipnoi, presaging the development of the distinct bulb of 

 the Amphibian heart. There are no vena cavae, but there 

 are two posterior cardinal vessels. 



12. The kidney is the persistent mesonephros. There is no distinct 



urinary bladder. (Small paired ones in some fishes.) 



Natural History 



Subclass Elasmobranchii. (Gr., a metal plate; Lat., a gill.) 

 Characteristics. — The Selachii have a cartilaginous skeleton, 

 placoid scales, gills covered (separate covers), heart with arterial 

 cone, a spiral valve in the large intestine and no swim bladder. 

 Sharks have triangular teeth with notched edges; dogfish have flat, 

 diamond-shaped teeth. They are marine except one fresh water 

 Nicauragan shark. (Figure 104.) 



The great white shark or " man-eater " {Carcharadon carcharius) 

 grows to thirty feet in length. Usually found in tropical waters, it 

 sometimes strays to the North Atlantic. It will follow ships and 

 seize refuse and so occasionally gets a " man overboard." Until the 

 summer of 191 6 all reports of attacks on man by sharks in American 

 waters were branded as false. Whale sharks reach a length of fifty 

 to sixty feet. They eat small fish, squids and shrimps, straining 

 them out of the water by means of the gill rakers. The thresher 

 shark has a much elongated upper lobe on its " super-heterocercal 

 tail," which furnishes a powerful weapon and also drives the small 



