DESICCATION IN PHILODINA ROSEOLA. 383 



sources of water is not marked and the same can be said of any 

 animal having a similar habitat and feeding habits. 



The production of metabolic water, in certain stages of the 

 life-histories of both plants and animals is sufficient for all 

 purposes for considerable periods of time. In the resting periods 

 of deciduous plants, in bulbs, in tubers and especially in seeds 

 and spores, ample water is provided for all vital processes by the 

 slow oxidation that takes place as a result of direct respiration. 

 Hibernating animals receive no water from external sources for 

 several months, although water is being constantly lost by 

 respiration and excretion. Many varieties of insects such as 

 clothes moths, grain weevils, dry-wood borers, etc., are able to 

 subsist during all stages of development upon air-dried food 

 materials containing less than 10 per cent, of water; in these 

 cases nearly all the water required is metabolic. 



Metabolic water may be formed as a result of two kinds of 

 respiration. In direct respiration the organic matter comprising 

 the food and tissues of an organism is oxidized by means of free 

 oxygen derived from the air during respiration. Many organ- 

 isms when deprived of free oxygen are capable of maintaining 

 for a short time certain of the respiratory functions, and deriving 

 energy from food material and from tissues by breaking up the 

 molecular structure into new forms of a lower order. This is 

 known as intramolecular respiration. 



When a Philodina dries it loses all its free water. The spaces 

 between the different organs are filled during the free-swimming 

 existence of the animal, with fluid. This fluid between organs 

 is the first to disappear. The loss of uncombined water is 

 responsible in large measure for the decrease in size of the animal 

 during the drying process. By examining Figs. I and 2 it will 

 be seen that in their normal condition the organs are not closely 

 packed together. It' is these spaces between organs which allow 

 the animal to decrease so appreciably in size and in such a 

 decrease the fluid content of the spaces is lost. 



In addition to the loss of free or uncombined water there is a 

 corresponding though not extensive loss of combined or chemi- 

 cally bound water. This is the imbibed water mentioned in a 

 previous paragraph. Just as water is distributed in the inter- 



