TORTOISES AND TURTLES. 39 



cesses which exchange the carbon dioxide of 

 the blood for oxygen. But there is this differ- 

 ence between the breathing of the tortoise and 

 that of the fish. In the former the vitiated 

 water is expelled through the mouth, in the 

 fish through one or more slits at the sides of 

 the throat. The water breathing of these tor- 

 toises is supplementary to that of the lungs, 

 and is effected in a perfectly rhythmical manner 

 some sixteen times per minute, by the move- 

 ment of the hyoid or tongue bones. Other 

 water tortoises, such as the European Pond- 

 Tortoise (Emys orbicular is), and the Sculptured 

 Terrapin of North America (Clemmys insculpata), 

 have developed accessory breathing organs in 

 the shape of thin-walled bladders which open 

 into the hinder end of the gut. The inside of 

 these bladders is richly ^supplied with blood- 

 vessels which are bathed by a constant exchange 

 of fresh water, the bladder being incessantly 

 filled and emptied through the vent. If one 

 of these tortoises is suddenly taken out of the 

 water the contents of these vessels will be 

 instantly squirted out. Although this habit is 

 well known, the source of the water is gener- 

 ally misunderstood, inasmuch as it is usually 

 supposed to be the urine from the bladder. 



Yet another extremely interesting modification 

 which the pond-tortoises have undergone is that 

 displayed by a large number of species forming 

 the sub-order Pleurodira. These all have ex- 

 tremely long necks, and on this account ap- 

 parently are unable to retract them so as to 

 the head within the shell. Instead, they 



