MODES OF REPRODUCTION 329 



paternal solicitude, we may suppose that the parent fish watched 

 over its eggs and took them into its mouth to remove them to 

 a place of security, and then gradually developed the habit of 

 retaining them till they were hatched. On the other hand it is 

 not impossible that the habit owed its first origin to a very 

 different intention on the part of the parent ; it is known that 

 many fishes eat the spawn of other fish and also their own 

 when they have the opportunity, and some of the eggs taken 

 into the mouth to be eaten may have escaped being swallowed 

 and remained in recesses of the pharynx till they were hatched. 

 We have seen that one of the most important conditions for de- 

 velopment is a constant supply of oxygen, and this condition 

 could be nowhere better fulfilled than in the pharynx of the 

 parent, through which water is constantly passing in the normal 

 respiration of the fish. As the two families are so remote it is 

 obvious that the habit must have been developed in each inde- 

 pendently, and they do not resemble each other closely in mode 

 of life : only a few species of cat-fishes have the habit, and 

 these are marine and estuarine, while it is common to all the 

 Cichlidae and they live exclusively in fresh water. 



The Cat-fishes which have adopted this mode of protecting 

 their eggs are species of Arius, Osteogeniosus and Galeichthys. 

 Ariits is cosmopolitan, species occurring in Asia, Africa, 

 America and Australia, but the habit is not followed by all the 

 species, Arius australis, for example, has been observed by Dr. 

 Semon to form nests in the Burnett River in Queensland. These 

 nests consist of circular excavations about twenty inches in 

 diameter in the sandy bed of the river ; the eggs are deposited 

 at the bottom of the hole and are covered over by several layers 

 of large stones. According to the well-known ichthyologist 

 Francis Day who was at one time Inspector-General of Fisheries 

 in India, all the Indian species of Arius carry their eggs in 

 their mouths ; the eggs fill the cavity of the mouth as far back 

 as the gills, and he never found any food in the stomach or in- 

 testines of fish carrying eggs in this way, so that during the 

 incubation the males are evidently unable to feed. Arius 

 commersonii is a large species occurring in South America, it 

 reaches three or four feet in length, and is the most commercially 

 important fish of the Rio Grande ; in the language of Brazil it 

 is known as the bagre ; it is dried for use as food, its bladder 



