In the nervous system the small size of the hindbrain or cerebellum is noticeable. The sense organs are fairly well developed. The skin of the whole body is provided with tactile nerve-endings. There are special taste organs on the lining membrane of the tongue and mouth-cavity. The eyes have no lids in some of the lower forms; most of the frogs and toads have an upper lid but no under one, although a thin membrane, called the nictitating membrane, arises from the lower margin of the eye and can be drawn up over it. The ears have no external parts, other than the thin tympanic membranes. The nostrils of frogs and toads can be closed by the contraction of certain special muscles. =Life-history and habits.=--The sexes are distinct, and in most cases the young hatch from eggs. A few of the salamanders give birth to free young. The eggs are usually in strings or chains enclosed in a clear gelatinous substance; these chains of eggs are either simply dropped into the water or are fastened to water-plants. The young, called tadpoles (fig. 120), in their earlier larval stages are extremely fish-like in character, long-bodied, tailed, swimming freely about by means of the fin-like flattened tail, and breathing by means of external gills. Nor do they show any sign of legs. As the tadpoles grow and develop the legs begin to appear, the hind legs first in the frogs and toads, the fore legs first in the salamanders; lungs develop and the gills disappear (except in the cases of the few forms which retain gills through life). The tail shortens and finally disappears in the frogs and toads; with the salamanders the tail-fin only is lost. At the same time the change from water to land is made. Further growth is very slow; frogs are not really adult, that is, capable of producing young, until they are five years old, and they may continue to increase in size until they are ten years old. [Illustration: FIG. 120.--Tadpoles. (Photograph from life by Cherry Kearton; permission of Cassel & Co.)] The food of the adult batrachians is almost exclusively small animals, particularly insects and worms. Crustaceans, snails, and young fish are also eaten. The tadpoles also eat vegetable matter. Almost all batrachians are nocturnal in habit, remaining concealed by day. In the zones in which cold winters occur they hibernate or pass the winter in a torpid condition, or state of "suspended animation," or, as it is said, they sleep through the winter. Frogs burrow into the mud at the bottom of ponds at the approach of winter and come forth early in the spring to lay their eggs. Most batrachians are very tenacious of life, being able to withstand long periods of fasting and serious mutilation, and most of them can regenerate certain lost parts, such as the tail or legs.