238 PRACTICAL ZOOLOGY 



through its walls. Undigested solids travel' onward into the 

 rectum and are cast out through the anus as feces. 



(2) The circulatory system transports the absorbed food to all 

 parts of the body. It also carries oxygen to the tissues, and 

 carbon dioxide and other waste products away from the tissues. 

 These substances are transported by fluids called blood and 

 lymph, which are usually confined in tubes, the blood vessels, 

 and in irregular spaces known as sinuses. The blood consists of 

 a plasma and corpuscles. It is forced to the various parts of the 

 body by the contractions of a muscular organ called the heart. 



(3) The respiratory system takes in oxygen (inspiration) and 

 gives off carbon dioxide (expiration). In many animals, like 

 the earthworm, the oxygen and carbon dioxide pass through the 

 moist surface of the body, but in higher animals there is a special 

 system of organs for this purpose. Aquatic animals usually 

 possess gills which take oxygen from the water. Terrestrial 

 animals generally take air into cavities in the body, such as the 

 lungs of man and the tracheae of insects. 



(4) The excretory system is necessary for the elimination of 

 waste products which are injurious to the body. These waste 

 products result from the oxidation of the protoplasm. Various 

 names are applied to the organs of excretion, such as nephridia 

 and kidneys. 



(5) The muscular system enables animals to move about in 

 search of food and to escape from their enemies. Many animals, 

 like the oyster, have the power of motion, but not of locomotion. 

 The muscles would be of slight efficiency were it not for the hard 

 skeletal parts to which they are attached and which serve as 

 levers. 



(6) The skeletal system is either external (exoskeleton) or 

 internal (endoskeleton). The hard shell of the cravfish is an 

 example of an exoskeleton; the bones of man form an endoskele- 

 ton. In either case the skeleton not only supports and protects 

 certain soft parts of the body, but it also provides places for 

 the attachment of muscles. 



