442 



LOWER VERTEBRATES. 



rated from each other by a bony septum. The internal ear is well developed, and the 

 nares open into the cavity of the pharynx, behind the palatine plate. 



The sight of most turtles is very keen, and they are generally watchful ; though 

 the marine forms are often captured while apparently asleep, their heads resting on 

 some floating piece of timber. They are also not unfrequently rendered partially 

 blind from the stings of the Portuguese man-of-war, PhysaMa, a coelenterate of 

 which they are very fond. The eyes are capable of being protected by not only the 

 ordinary upper and under lid, but also by a third, like that of birds, a lateral nictitat- 

 ing membrane. Hearing in some forms is acute ; the fact that many, if not all, are 



Fig. 256. — Sp?iargis coriacea, leather-back turtle. 



musical, argues that their organs of hearing are well developed. The tympanum is at 

 the side of the head, and, like that of the frogs, is unprotected by either valves or 

 cavity. The senses of smell and taste, allied to each other, are but poorly developed. 



The circulatory, digestive, and reproductive organs generally resemble those of the 

 lizards proper, though the Crocodilia are anticipated, and the birds to some extent 

 foreshadowed. Kespiration is effected by the swallowing of air, the rigid carapax 

 and plastron effectually preventing the expansion of the chest. All turtles are ovi- 

 parous. 



While some of the families of the order of which we are now to treat contain gen- 

 era which may be enumerated by tens, and species by hundreds, the more embryonic 



