CONDITIONS OF LIFE 299 



great, as for example in the case of the sole. There is evidence, 

 however, that young plaice remain buried in the sand in the 

 shallow water in winter. On the shallow grounds off the Dutch 

 and German coasts of the North Sea small plaice are taken in 

 enormous numbers in spring and summer, but are not to be 

 caught with the trawl in January, February and March, and it 

 was observed in the aquarium at Cleethorpes in Lincolnshire 

 that such small plaice never showed themselves in the cold 

 weather, but emerged from the sand and were eager for food 

 the first warm day in spring. ^Estivation or the assumption of 

 a torpid condition in the dry season is not uncommon among 

 the fresh-water species of the tropics. The lung-fishes (Dipnoi) 

 Protopterus in Africa and Lepidosiren in South America, 

 living in swamps which dry up in the rainless part of the 

 year, burrow into the mud and remain torpid during the dry 

 season, while Ceratodus, inhabiting the deep water-holes of 

 the rivers of Queensland which seldom or never dry up com- 

 pletely, does not aestivate in this manner. Protopterus coils 

 itself up at the bottom of its burrow and becomes surrounded 

 by a layer of slime secreted by the skin ; the slime hardens and 

 forms a closed capsule perforated only by a hole opposite the 

 fish's mouth for the entrance of air to the lungs. Specimens of 

 this fish have been frequently imported into England, encased in 

 their cocoons of dried mud and slime. On arrival the mass of 

 mud has been softened with warm water, and the liberated fish 

 has revived and lived in an aquarium for months or years. 

 Lepidosiren makes a similar burrow but the mouth of it is closed 

 by a lump of clay perforated by several holes. It is interesting 

 that those fishes of the Order Teleostei which aestivate in a 

 somewhat similar way have also organs for breathing air, and 

 these must have been evolved in response to similar needs in 

 similar modes of life ; but in these the organs are not lungs or 

 air-bladder but are accessory organs connected with the gill- 

 chamber. The Ophiocephalidae or serpent-heads are abundant 

 in India and Ceylon, and also in China and tropical Africa ; 

 when the water of the pond or " tank " in which they are living 

 dries up they simply bury themselves in the mud and remain 

 there till the drought is passed. The climbing perches (Ana- 

 bantidas) and gouramis (Osphromenidae) survive in the mud of 

 dried up ponds in the same manner. Among the cat-fishes 



