DIVISIONS OF ZOOLOGY. 1] 
4 parts of Ammonia(NH;) ... te yee 
7 . Water (H, O).. A ys O; 
36 a Carbonic Acid (C O,) Art Ores te 
Total eae eee an Hea N, ce. 
1 part Protein ae a 3 10 
Oxygen liberated ... Ons 
Consequently, for every molecule of protoplasm formed, 69: 
atoms of oxygen are liberated. When, therefore, this proto- 
plasm is decomposed back into the substances from which it 
was formed, an equal amount of oxygen must be absorbed to 
convert the carbon into carbonic acid, otherwise the carbon 
would accumulate in the animal and destroy it. This oxygen 
is introduced into the animal by the respiratory organs,. 
which also get rid of the carbonic acid. The ammonia is 
removed by the renal organ. <A respiratory organ consists 
essentially of a thin membrane, to one side of which is 
applied the fluid containing oxygen, to the other, the blood of 
the animal that has to be oxygenated. If the fluid containing 
oxygen is water, the membrane projects outward as a fold 
into it, and is called a gill, or branchia. If the fluid is air, 
the membrane forms an involution, or sac, in the interior of 
the animal, and is then called a pulmonary vesicle, or 
lung. If this sac is prolonged into tubes, carrying the air 
to all parts of the body, then the tubes are called trachee. 
Those parts of the animal in which the reproductive 
elements are developed are called the reproductive organs, 
or sex glands. The female organs produce ova, which have 
been already described (p. 6); the male organs produce 
spermatozoa, which are usually long filiform bodies with a 
more or less thickened head and have active movements, the 
long body being waved backward and forward. In some 
cases, however, as in some crustacea, the spermatozoa are. 
