AND PHYSIOLOGY. 



313 



thus making each feather an impenetrable plans, surface. 

 The feathers also overlap one another to a considerable de- 

 gree, and as they thus contain air, and are themselves non- 

 conductors, they afford the most perfect protection to the body 

 against the cold. 



o 



555. Skin of Amphibia, — The naked Amphibia, such as 

 the Frogs, have a smooth slippery skin, which is continually 

 being cast off in patches or shreds. This skin surrounds the 

 solid parts of the body very loosely, and spaces for lymphatic 

 vessels are found beneath. It is always composed of several 

 layers of fibres which lie at right angles with each other. 



556. Scales of Serpents . — Upon some serpents, scales are 

 found which overlap one another, like the shingles of a house, 

 as is also the case with fishes. Scales with tubercular spines 

 are met with on some reptiles, and in some instances, as those 

 of Crocodiles and Tortoises, they contain bony matter, and 

 coalesce with the bones of the skeleton. 



557. Exuviation of Serpents. — Many Serpents cast their 

 skin several times during the year, either by piecemeal, or by 

 drawing off the whole at once. One species of Tortoise d^es 

 the same thing, as well as several Lizards. This is an, anal- 

 ogous function to the moulting of birds, and shedding of the 

 coat in mammals. This generally takes place in Spring, 

 but frequently upon a change of weather several times in the 

 year. At each period, when this is effected by the Rattle- 

 Snake, a new segment is said to be added to the tail, which 

 seems quite probable, since the rattle is merely a condensed 

 portion of the epidermis. 



558. Scales of Fishes. — Classification of Fishes by 

 their Scales, — Fishes have an Epidermis which is sometimes 

 covered with scales, and sometimes not, though always lubri- 

 cated most thoroughly by a copious mucous secretion. The 



555. What is said of the skin of frogs ? 553. Do serpents ever have scales? 557. What 

 is the exuviation of serpents? When does this take place? What peculiarity in the 

 rattle-snake ? 558. How are the scales of fishes arranged ? 



