MOST people are familiar with the development of the common frog, which in this respect may well be taken as representative of the higher amphibians. The eggs are
deposited in water and the thin layer of albumen, with which each egg becomes covered in passing from the bod)’ of the parent, swells in contact with the water, so that all the eggs stick together and form a jelly-like mass to be hatched by the heat of the sun.
At first the young frog is without limbs or gills and it soon develops into what is known as a ‘tadpole.’ At this stage it is a small, limbless creature, with a long swimming tail, and provided with two discs behind the mouth by means of which it can attach itself to foreign objects. It next develops outward gills on the sides of the neck, and also gill clefts leading into the throat, and is now breathing air like a fish, that is by taking in water at the mouth and passing it out by the gill clefts. At this point it lives on vegetable food and has no limbs.
In the next stage the outer gills disappear and the construction of the heart becomes more complicated. The tail now shortens and is absorbed into the body, the mouth becomes wider, teeth are developed, whilst the body goes on growing. Finally the limbs develop, the tail is completely lost, and the perfect frog emerges.
The adult animals breathe air only, their lungs being comparatively well developed. The skin also helps, and frogs can undoubtedly breathe through their skins for a long time without needing to use their lungs. A frog when breathing actually goes through the process of swallowing, and consequently he can be suffocated by forcibly holding his mouth open and preventing him passing air down his throat by the contraction of the cheek muscles.
In the majority of frogs and toads the tongue is fixed in front and free behind. This is so in the case of the Common Grass Frog, very widely distributed, and of the Edible Frog, which is almost equally frequent. In this latter species the males are furnished with a vocal sac on each side of the head which increases the resonance of the voice. They are far outdone in this respect, however, by the Great Bull Frog of North America—the largest species of all—whose voice is like the bellowing of a bull, and is audible at a distance of half a mile. The Tree Frogs have the ends of their toes expanded into the form of sucking discs. The only European species is the little Green Tree Frog, which is a bright emerald in colour and has a loud, piping voice. It spends the summer in trees searching for insects, but in the winter retires to the water to spawn and hibernate.
WHERE THE FROG AND TOAD DIFFER
TOADS are a very widely distributed family, though of about a hundred known species only three are European. Two of these are British, the Common Toad and the lesser-known Natter Jack Toad. Toads differ from frogs in having no teeth; their legs are shorter and stouter, the head is more blunt, the skin very warty, and the hind feet are only partially webbed. Toads do not lay their eggs in a mass but in a string, and the tadpole’s are smaller than those of the frog.
They are often kept in gardens where they prove themselves very useful by devouring countless insects. They have been the subject of many superstitions on account of their appearance but the beauty of their eyes does much to redeem their undoubted ugliness. Their only means of defence is to exude a bitter moisture from the skin, which will often prevent a dog from seizing them in his mouth.