Sio 



FISHES 



ditions, there can be little doubt that its value as such is much 

 greater whenever gill-breathing becomes difficult or impossible. 

 This seems to be the case during the hot season, when the water 

 becomes foul from the presence of decomposing animal or vegetable 

 matter. Semon records a striking illustration of this in the 

 case of a partially dried -up water-hole, in which the water had 

 become so foul that it was full of dead fishes of various kinds. 

 Fatal as these conditions were to ordinary Fishes, Neoceratodus 

 not only survived but seemed to be quite healthy and fresh. 

 Such observations are of exceptional interest. Not only do they 

 afford a clue to the conditions of life which, in the course of 

 time, probably led to lung-breathing in Neoceratodus, but they 

 also suggest the possibility that a similar environment has been 

 conducive to the evolution of air-breathing Vertebrates from gill- 



I>-^ 



Fig. 305. — A young Neoceratodus four weeks after hatching, c, Cloacal aperture ; 

 l.l, lateral line ; 7ft, mouth ; op^ operculum ; 2).f, pectoral fin. (From Semon.) 



breathing and Fish-like progenitors. In spite of its pulmonary 

 respiration, Neoceratodus more closely resembles the typical Fishes 

 in its habits than any other Dipneusti. It lives all the year 

 round in the water. There is no evidence that it ever becomes 

 dried up in the mud, or passes into a summer sleep in a cocoon, 

 and the well-developed condition of its gills suggest that these 

 organs play a more important role in breathing than in either 

 ProtO'pterus or Zepidosiren,. The Fish is not known to leave the 

 water, and the paired fins, useful no doubt as paddles, are quite 

 incapable of supporting the bulky body on terra firma. In fact, 

 when Neoceratodus is taken out of its natural element it seems 

 to be more helpless than most other Fishes, and, in spite of its 

 capacity for lung -breathing, soon dies unless kept moist by 

 artificial means. Spawning takes place from April to November, 

 principally in September and October. The eggs, invested by 

 a jelly-like coat, secreted by the oviducal walls, are deposited 



