234 GENERAL ZOOLOGY 



which they capture and devour. The sting ray (Fig, 91, 

 C) often inflicts painful wounds on the bodies of bathers 

 and fishermen. It has a barbed poison spine at the base 

 of its tail which is driven with great force into any animal 

 that molests it. A tropical ray, the torpedo, has electrical 

 organs in its body which are under the control of the nerv- 

 ous system. These are capable of giving powerful shocks 

 and serve as effective weapons of defense or offense. 



The placoid scales (Fig. 93) of elasmobranchs are of par- 

 ticular interest because their structure closely resembles 

 that of the teeth in the mouths of true fishes, amphibians, 

 reptiles, and mammals. Each is covered with a layer of 

 enamel, which surrounds a bony dentine layer, and the 

 interior is filled with "pulp" containing blood vessels, 

 nerves, and connective tissue. It is believed that the teeth 

 of vertebrates have been derived from placoid scales such 

 as the sharks and rays possess. In elasmobranchs these 

 organs occur in the skin over the whole body and in the 

 mouth. Those in the mouth are on a portion of the ecto- 

 derm which has been turned inside; they are particularly 

 large and arranged in regular rows. In other vertebrates 

 the teeth are restricted to the mouth, or in other words/ 

 placoid scales are lacking from the skin. 



