46 



STRUCTURE OF THE VERTEBRATES 



animals burrow into the mud and aestivate, a condition physi- 

 ologically similar and seasonally opposite to hibernation. As the 

 water in the lakes recedes the fish burrows into the ground, 

 secretes a mucous capsule around itself, and remains there un- 

 til the return of wet weather. Normally this is not more than six 

 or seven months, although specimens have been kept in the 

 laboratory encased in dry mud for more than eighteen months 

 without any marked loss in weight. On being broken out of the 

 capsule and placed in water, respiration begins in a few seconds, 



Lepidosiren larva 



Lepidosiren (Dipnoi) 



Fig. 16. Recent Lung-fish. Ceratodiis is more primitive. Lepidosiren has 

 degenerate fins and an elongated body. The specimen illustrated has the 

 vascular outgrowths which appear on the pelvic fins of the male during 



breeding time. 



and the fish is apparently none the worse for the experience. 

 These modifications have gone so far in the South American 

 form that the animal is incapable of supplying itself with suffi- 

 cient oxygen through its gills, and drowns when kept immersed 

 in water. During the breeding season the male of this genus 

 (Lepidosiren) develops accessory gills on its pelvic fins, and is 

 thus able to remain under water and guard the eggs. 



