AMPHIBIA 



295 



not well developed and the ear drum and middle ear are wanting. This 

 is a small group, found in tropical regions living underground. 



639. Order 2. — The Urodela have an elongated body with a tail and 

 usually weak legs. In one family (Sirenidae) the posterior legs are 

 wanting. In some families the gills are retained in the adult and in 

 others the gills are lost but the gill slits are retained. In most cases, how- 

 ever, the gills and slit both disappear. The eyes are small and the ear 



Fig. 173. — Diagram of a bird embryo within the egg membrane. The foetal 

 membranes are omitted (see next figure). 6, Brain; h.w., body wall; c.c, central 

 canal of spinal cord; co, coelom; g, intestine; g.iv., wall of intestine; s.c, spinal 

 cord; y.s., yolk-sac. 



drum and middle ear are absent, To this order belong newts, efts, 

 "spring lizards," "mud puppies" and salamanders. 



640. Order 3. — The Anura comprise frogs, toads and tree toads. The 

 body is short and tailless. The posterior pair of appendages are usually 

 long and strong. The eyes are large; there is usually an ear drum with 

 a middle ear communicating with the mouth cavity. 



641. Class IV. Reptilia.^ — The Reptiles are typically terres- 

 trial though many live in the water. They never have gills. 

 The skin is covered with horny scales or plates formed by the 



