156 



PHYSIOLOGY 



may be developed to such an extent that an animal may 

 bury itself deeply in mud or in piles and yet get the 

 necessary food and oxygen (Fig. 90). 





Fig. 87. -Various forms of shells. (From Hertwig.) 



Gills. — In the space between the mantle and the body 

 extend on each side two other folds of skin called 

 gills. They are very thin and are permeated with thin- 

 walled blood vessels by means of which blood is brought 

 into connection with oxygen dissolved in the water. 



