THE STUDY OF ORGANS 39 



glandular lining of the intestine, which is called the 

 succus entericus (intestinal juice). 



There are other glands besides the digestive glands; 

 but all consist, like these, of highly active cells, which 

 make some special substance, and either pour it direct 

 into some special organ, or transfer it by a special 

 tube (duct) to its destination. 



The alimentary canal is sometimes provided with an 

 appendage called a csecum, because it consists of a 

 tube with no opening (i.a. blind) at the far end. This 

 is another arrangement for securing the greatest area 

 of digestive surface with the least possible waste of 

 space. The rabbit has a very large csecum. 



The Xiung^S of vertebrates are also formed from 

 the hypoblast. They consist of a complicated system 

 of branching air-chambers lined by a continuation 

 of the same epithelium. No organ perhaps is so 

 variable throughout the animal kingdom as the 

 breathing organ. While the higher vertebrates have 

 lungs, the lower ones have gills, and the intermediate 

 forms (A.mphibia, frogs and newts) have gills first 

 and lungs afterwards. The lung is not developed 

 from the gills, but is an independent organ-situated 

 in quite a different place. Among the Mollusca 

 or shell-fish, the marine shell-fish' have gills, and 

 the terrestrial shell- fish lungs in a corresponding 

 place. Among the segmented animals, insects have 

 breathing-tubes opening in a row on their sides, 

 while the aquatic larvie of some insects^ breathe 



' ^.g. dragon-fly and gnat. 



