THE ALIMENTARY CANAL. 



75 



be said to have a digestive canal. The animal is here 

 composed of a single cell, in which the food is digested. 

 The jelly-like Amoeba passes the food through the firmer 

 outer layer (ectosarc) into the more fluid inner part (endo- 

 sarc\ where it is digested. The Infusoria, which have a 

 cuticle, and so a more definite form, possess a mouth, or 

 opening, into the interior of their cell-body, and at least 

 a definite place where the excrement is passed out. But 

 we cannot call this cell-cavity a digestive tract. 



In the higher animals, the alimentary canal is a contin- 

 uation of the skin, which is reflected inward, as we turn 

 the finger of a glove. 35 We find every grade of this re- 

 flection, from the sac of the Hydra to the long intestinal 

 tube of the Ox. So that food in the stomach is still out- 

 side of the true body. 



The simplest form of such a digestive tract is seen in 

 the Hydra (Fig. 191). Here the body is a simple bag, 

 whose walls are composed of two layers of cells (ectoderm 

 and endoderm). A 

 mouth leads into the 

 cavity, and serves as 

 well for the outlet of 

 matter not wanted. 

 The endodermal cells 

 furnish the juices by 

 which the food is di- 

 gested and absorb the 

 nutritious portions of 

 it. There is no rad- 

 ical difference, how- 

 ever, between the two 



lsvmvs of r>ollc -f™ f FlG ' 38.— Dissected Actinia: a, the thick opaque skin 



iay tu& ui btJiite, 1UI tut; consisting of ectoderm, lined with muscular fibres ; 



Hydra has been turn- c ' tne tu ^ ular tentacles communicating with the in- 



" terspaces ; Jc, between the membranous vertical 



ed inside OUt, when folds; g, g', orifices in the walls allowing passage 



. ' p of respiratory water from one compartment to an- 



the IOrmer ectoderm other; mouth leading to gastric cavity, e. 



