352 COMPARATIVE ZOOLOGY OF CHORD ATES 



pterus and Lepidosiren among the Dipnoi, by Amia, several 

 catfish, and the stickleback, only to mention a few. In some 

 of these cases there are interesting adaptations for ensuring a 

 sufficient supply of oxygen to the eggs. So in Lepidosiren, 

 the pelvic fin of the male becomes modified into a tuft-like 

 organ well supplied with blood, from which oxygen diffuses 

 out into the water. In some catfish, the eggs are carried about 

 by the parent (usually the male), and so are continually exposed 

 to fresh sea water. Ichthyophys (Gymnophiona) coils itself 

 round its eggs in a burrow, as do some snakes such as the 

 python. Several Anura lay their eggs in nests specially 

 prepared ; others make living nests of themselves. In Pipa 

 the eggs are placed on the female's back, where they sink into 

 pits and undergo development ; the male Rhinoderma carries 

 the eggs in large vocal sacs ; Hylambates carries the eggs in 

 its mouth. Alytes is peculiar in that pairing takes place on 

 land, and the eggs, which are tied together by strings of slime, 

 are carried about by the male, wound round his legs. When 

 the young are about to hatch, the male takes them to the water 

 and abandons them. In some viviparous snakes such as the 

 viper, the young remain with the parent for a time. It 

 happens in some forms (e.g. viper) that the egg is hatched 

 while still in the oviduct, without being laid. This condition 

 is called ovo-viviparous. 



The eggs of birds require a constant high temperature for 

 their development, and this necessitates the uninterrupted 

 attention of the parents. (In the Megapodes, the eggs are 

 laid in heaps of decaying vegetable matter, the heat of which 

 enables them to incubate.) Nearly all birds lay their eggs in 

 nests, which serve to protect the eggs from enemies, cold and 

 damp. Nests are constructions in which as a rule both sexes 

 take part, and courtship and display play an important part in 

 keeping together the members of a pair to perform the various 

 duties which devolve on them. The male is usually the 

 active partner in courtship, and often possesses brilliant 

 secondary sexual characters used for the purpose. After the 

 nest has been built, these duties include the collecting of food 

 for the sitting partner, and for the young when they hatch in 



