36 AN INTRODUCTION TO ZOOLOGY. 



brata; of these perhaps the most remarkable is the 

 "compound eye" of insects. 



Parts derived from the Hypoblast. The 

 lining of the digestive tube, thi-ough which food passes, 

 consists of a layer of cells called epithelium in 

 the adulb animal, and hypoblast in the embryo. It 

 is the digestive layer. The tube itself is called the 

 Alim.entary Canal. ^ In simple animals it is 

 merely a straight tube. In the higher vertebrates 

 it is immensely complicated. Its chief parts are the 

 stom.ach, or gastric region, which is adapted for 

 the digestion chiefly of proteid foods (see p. 8), and 

 the intestine, a portion adapted for the digestion of 

 all sorts of food. This is convoluted or straight, ac- 



' It is also spokea of as the Gut, in which three regions are 

 distinguished — the Pore-gut, Mid-gut, and Hind-gut. The 

 use of these terms is chiefly restricted to embryology. 



It is usually understood that the mid-gut comprises the 

 true alimentary canal lined by hypoblast, while the fore-gut 

 and hind-gut are formed by inlets from the epiblast, which 

 form respectively the mouth and excretory opening (anus), 

 and are called the stomodaeum or stomatodaeum and procio- 

 daeum (Gk. ord/ia, mouth ; npmKTos, anus). The extent of 

 these regions varies greatly in different types of animal, and 

 hence there is some ambiguity in the use of the terms fore- 

 gut and hind-gut. It may be noted that the use of those 

 terms is an instance of the reaction which has of late taken 

 place against the exclusive use in scientific nomenclature of 

 words derived from the Latin and Greek. In imitation of the 

 example of the Germans, an attempt is sometimes made to 

 utilize popular English names ; but the possibilities of their 

 use are very limited, and even where they can be used the 

 effect is not always happy. 



