HABITS. 



257 



and the blastopore persists as the anus. In Ceratodus the central 

 nervous system arises by the closing in of a groove as in most 

 vertebrates. In Lepidosiren the medullary canal has at first 

 the form of a solid keel-like thickening of the ectoderm, as in 

 Lepidosteus and Teleosteans. The young are hatched as larvae 

 which possess external gills in Lepidosiren, but not in Ceratodus. 

 Growth probably lasts for some time if not throughout life, and 

 a length of three feet may be attained. It is quite probable 

 that during this long period of growth the structure of the 

 animal may change considerably, and that contradictory 

 anatomical results may be explained in this way. It is probable 

 that the fins can be regenerated. 



In Proiopterus there does not appear to be any marked external 

 difference between the sexes, but in Lepidosiren the male 



Fig. 134. — A larva of Lepidosiren 25 days after liatchiiig, showing the external gills,[the 

 rudiments of the fore and hind limbs, and the cloacal aperture cl.o (after Kerr).j 



acquires during the breeding season a large tuft of long highly 

 vascular papillae on its pelvic fins. Both these genera breathe 

 air when living in water, but the respiratory movements are 

 less frequent than in the cocoon form. 



Ceratodus is not a mud-fish. It never leaves the water. It 

 is found in the dry season in the deep pools of the rivers in which 

 it lives. These pools become very foul when the river shrinks 

 in the hot season, and no doubt under these conditions the 

 branchial respiration is largely supplemented by pulmonar3\ 



Lepidosiren and Protopterus are true mud-fish. They bury 

 themselves at the beginning of the dry season in the mud of the 

 swamps in which they live. As the drying mud stiffens they 

 retire deeper into the burrow, an opening always remaining at 

 the upper end for respiration. In this burrow the Protopterus 



