THE ALIMENTARY CANAL. 



75 



The Protozoa, as the Amoeba and Infusoria, can hardly 

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

 composed of a single cell, in which the food is digest- 

 ed. The jelly-like Amoeba passes the food through 

 the firmer outer layer (ectosarc) into the more fluid 

 inner part (endosarc), where it is digested. The Infu- 

 soria, which have a cuticle, and so a more definite form 5 

 possess a mouth, or opening, into the interior of their 

 cell-body, and at least a definite place where the excre- 

 ment 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. 36 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 (ecto- 

 derm and endoderm). 

 A mouth leads into 

 the cavity, and serves 

 as well for the out- 

 let of matter not 

 wanted. The endo- 

 dermal cells furnish 



, i . , i . i PIG. 38. Dissected Actinia : or, the thick opaque skin 



B ]UlCeS Dy wniCl consisting of ectoderm, lined with muscular fibres; 



flip fnnrl is rHo-pQtprl c i tne tubular tentacles communicating with the 

 interspaces, *, between the membranous vertical 



and absorb the nil- folds; g, g', orifices in the walls allowing passage 

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



tritlOUS portions OI other; d, month leading to gastric cavity, e. 



