LESSONS IN ZOOLOGY. 255 



part of it the stomach with its glands and accessory organs is 

 formed. From the middle part, that portion of the intestine 

 in which digestion is completed, by the action of fluids from 

 the pancreas, liver, and intestinal glands, is derived, and from 

 the hind part that portion of the intestine concerned in the 

 collection and expulsion of undigested portions of food had its 

 origin. 



The liver and pancreas appear to be outgrowths of the alimen- 

 tary canal which have become glands. The liver, distinguished 

 for its relatively great size and weight in the higher animals, 

 has been developed from a little patch of colored cells. The se- 

 cretion of the liver sometimes aids in digestion and sometimes it 

 does not. 



Among the lowerformsof animals no circulatory system is nec- 

 essary, as the digestive cavity extends to all parts of the body ; 

 but in animals of a little higher grade the chyle or nutrient 

 fluid circulates in the space between the wall of the digestive cav- 

 ity and the body wall, doubtless due to the ordinary motions 

 of the organism; later a particular portion of this tract ac- 

 quires a muscular structure, which as a special organ maintains 

 the circulation. Between this and the double heart of the mam- 

 malia, with its systems of arteries and veins, all gradations may 

 be found. 



The respiratory organs are appendages of the circulatory sys- 

 tem, arising from arches of the greater arteries, gradually devel- 

 oping from simple tubes, the gills of fish and the tracheae of insects, 

 to the capacious lungs of the warm-blooded mammalia. 



The respiratory organs are to some extent excretory organs, 

 but the most important excretory organs are the kidneys. In 

 the lowest animals they are represented by contractile vacuoles; 

 in the worms they are known as water vascular vessels, and 

 sometimes they are mere appendages of the lower intestine, while 

 in the higher animals they are independent organs made up of a 

 great number of narrow coiled tubes with an abundance of blood 

 vessels. They separate from the blood broken down albuminous 

 matters. The skin is also in some sense an excretory organ, while 



