22 



THE FROG 



The common frog of Britain is the species known in zoology 



as Rana temporaria. It is abundant in summer in damp places, 



but in winter is less easily found, owing to the fact that it is then 



in a torpid state, hidden in holes or buried in mud. In the spring 



the warmth wakes the frogs and they congregate, croaking 



loudly, and pair in the water, where the eggs are laid, enclosed 



in jelly as a mass of spawn and fertilised by the sperms which the 



male sheds over them as they pass out of the female. In about 



a fortnight there hatches from each e^g a little, fish-like tadpole. 



This has no limbs, but a strong tail which it uses for swimming, 



breathes wholly by gills, and is at first without a mouth. A young 



animal which like a tadpole or a caterpillar differs from the 



adults of its kind in possessing special features of its own, but 



is capable of fending for itself, is known as a larva. The term 



embryo is applied to a young organism while it is helpless and 



is developing within the body of its parent or under shelter of 



an egg-shell or a jelly coat, like the young of man or a bird, or 



the early stages of the frog. In a few days the tadpole comes to 



possess a mouth and begins to feed mainly on small particles 



which are entangled in mucus and carried along by cilia, in the 



same way as the food of Branchiostoma (p. 304). Some of these 



particles are rubbed off plants by a buccal rasp in the mouth, 



and larger particles, such as water- fleas and insect larvae, are 



also swallowed. Gradually it changes, losing its gills and tail 



and gaining lungs and two pairs of limbs, till at the end of three 



months it becomes a small frog. Henceforward it lives principally 



on land, sometimes crawling about by means of both pairs of 



limbs, but generally jumping with the strong hinder pair and using 



the small fore pair to break its fall when it alights. From time to 



time, however, it takes to the water, and then it can swim 



strongly with its hind limbs. Its food, after it has left the water, 



consists largely of insects and molluscs with worms and other 



small animals, the smaller prey being caught by a sticky tongue, 



the larger seized with the mouth. 

 M.z. — 12 345 



