AMPHIBIA 249 



complete crystalline lens is formed in the new situation. 

 The presence of optic tissue stimulates the ectoderm to 

 form a lens. Metabolic processes have much more latitude 

 in fishes and amphibians than in warm-blooded vertebrates 

 where powers of regeneration are limited and growth 

 processes are not open to much modification. 



The mud-puppy is often attacked by parasites and in- 

 fectious diseases. Fish-mould (Saprolegnia) , a fungus which 

 enters the body through slight abrasions and destroys the 

 tissues, is the most important of these. Another common 

 parasite is a minute trematode worm which attaches itself 

 to the gills and sucks blood. 



Necturus remains active beneath the ice throughout the 

 winter, in this respect resembling fishes and some other 

 aquatic animals which dwell in lakes and rivers. The 

 temperature of the water may be low, but never falls 

 below freezing, and conditions are, therefore, stable enough 

 to permit a sluggish, but not wholly inactive, existence. 



Race Preservation. Amphibians are always dioecious, 

 but Necturus males are much like the females in appear- 

 ance. Though animals are usually found in pairs in 

 autumn, egg laying takes place in May and June. Each 

 female deposits a number of eggs enclosed in little cases 

 and attached by stalks to the under side of stones (Fig. 

 96, D), logs, tin cans, and other objects in the water, where 

 they pass through the usual developmental stages (Fig. 

 65). The whole egg separates during cleavage (Fig. 115), 

 but at one pole the accumulation of yolk retards cell-forma- 

 tion somewhat and the cells are, therefore, larger there. 

 Instead of forming on one side of the egg, as in fishes, the 

 embryo develops entirely around the yolk-containing por- 

 tion, but there is a large yolk sac which protrudes from the. 

 ventral side of the developing embryo until absorbed 

 (Fig. 97, E). 



The young (Fig. 97, A) are striped along the back, and 

 thus differ from the adults. They live in the water plants 

 alongshore where they are effectively concealed by their 



