78 COLLEGE ZOOLOGY 



juices from the salivary glands; the mixture then passes through 

 the oesophagus into the stomach, where chemical digestion, aided 

 by secretions from the gastric glands, takes place; it then enters 

 the intestine, which absorbs the dissolved material through its 

 walls. Undigested solids travel onward into the rectum and are 

 cast out through the anus as faeces. 



(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 muscular organs called hearts. 



(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 vertebrates and the trachea of insects. 



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

 the waste products of metabolism which are injurious to the 

 body. These waste products result from the oxidation of the 

 protoplasm. Various names are applied to the organs of excre- 

 tion such as nephridia (Fig. 153, neph.) and kidneys (Fig. 417). 



(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 {endo skeleton) . The hard shell of the crayfish is an 



