老人越来越瘦是什么原因

百度 境外当地体验类产品的当天预订量占比增长了10%，境内则增长了13%。

Reptiles Temporal range: Pennsylvanian–present, 312–0 mya
Clockwise from above left: Green sea turtle (Chelonia mydas), Tuatara (Sphenodon punctatus), Nile crocodile (Crocodylus niloticus), and Sinai agama (Pseudotrapelus sinaitus)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Sauropsida
Class:	Reptilia Laurenti, 1768
Living groups
Lepidosauria (lepidosaurs) Rhynchocephalia (tuatara and relatives) Squamata (lizards and snakes) Testudines (turtles) Archosauria (archosaurs) Crocodilia (crocodilians) Aves (birds) (included in cladistic analyses, but excluded in traditional taxonomy) See text for extinct groups.
Global reptile distribution (excluding birds)

Reptile is the common name for one of the main groups of land vertebrates. It is not used so much by biologists, who use more accurate terms.

The name "reptile" comes from Latin and means "one who creeps". All living reptile species are cold blooded, have scaly skin, and lay cleidoic eggs. They excrete uric acid (instead of urea), and have a cloaca. A cloaca is a shared opening for the anus, urinary tract and reproductive ducts. Reptiles also share an arrangement of the heart and major blood vessels which is different from that of mammals. Birds have all of these features.

Many important groups of reptiles are now extinct, for example the mosasaurs. We used to say the dinosaurs were extinct, but they survive in the form of their feathered descendants (birds). Ancient reptiles that do survive include the turtles, the crocodiles and the Tuatara, the lone survivor of its group. The great majority of present-day reptiles are snakes and lizards.

The study of living reptiles is called herpetology.

Birds in relation to reptiles

Some reptiles are more closely related to birds than they are to other reptiles. Crocodiles are more closely related to birds than they are to lizards. Theropod dinosaurs are even more closely related, because birds evolved from them.

Cladistic writers prefer to put the birds (over 10,000 species) with what people usually call reptiles. (see Sauropsida)

Taxonomy

Reptilia is an evolutionary grade rather than a clade. The main reason is that the term 'reptile' does not include birds, the descendents of theropod dinosaurs. Another reason is that the word 'reptile' is misleading because many extinct types were very different from living reptiles.

So instead of Reptilia as a taxonomic class, today many experts use Class Sauropsida (which includes all reptiles and birds, living and extinct). Class Synapsida includes mammals and all their forebears. Reptile is still the usual informal term to describe living snakes and lizards. Mammals are a genuine clade, and so Mammalia is still the taxonomic term.

Since reptiles are not monophyletic, reclassifying them is one of the key aims of researchers. Some taxonomists, such as Benton, make Sauropsida and Synapsida class-level taxa. The two groups split in the Carboniferous, from stem-group Amniotes (the early tetrapods, which laid cleidoic eggs).

Eye feature

A membrane forms an inner eyelid in reptiles and birds. Whitish or translucent, it can be drawn across the eye to protect it from dust and keep it moist. It is called the nictitating membrane.

Size

Reptiles can live in large and small sizes. Their land sizes can be both bigger and smaller than mammals. Titanosaurs were the largest land reptiles, and the smallest land reptile is a chameleon 13.5mm long.

Living reptiles

• Reptiles (living groups only)
  • Order Crocodilia (crocodiles, gavials, caimans, and alligators): 23 species
  • Order Sphenodontia (Tuatara from New Zealand): 2 species
  • Order Squamata (lizards, snakes, and amphisbaenids ("worm-lizards"): about 7,000 species.
  • Order Testudines (turtles, tortoises, and terrapins): approximately 300 species

• 
  Crocodile
• 
  Tuatara
• 
  Turtle

• 
  Chameleons, a lizard
  
• 
  Snake
• 
  Komodo dragon, sleeping. Largest living lizard