THE ALIMENTARY CANAL 



277 



which have a cuticle, and so a more definite form, pos- 

 sess a mouth, or opening, into the interior of their cell 

 body, and at least a definite place where the excrement 

 is passed out (Figs, 9, u). But we can not call this 

 cell cavity a digestive tract. 



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

 tinuation of the skin, which is reflected inward, as we 

 turn the finger of a glove. 89 We find every grade of 

 this reflection, from the sac of the hydra to the long in- 

 testinal tube of the ox. So that food in the stomach is 

 still outside of the true body. 



The simplest form of such a digestive tract is seen in 

 the hydra (Fig. 18). Here the body is a simple bag, 

 whose walls are 

 composed of two 

 layers of cells (ecto- 

 derm and endo- 

 denri). A mouth 

 leads into the cav- 

 ity, and serves as 

 well for the out- 

 let of matter not 

 wanted. The en- 

 dodermal cells fur- 

 nish the juices by 

 which the food is 

 digested and ab- 

 sorb the nutritious 

 portions of it. The 

 polyps have also 

 but one external opening ; but from this hangs down a 

 short tube, open at both ends, reaching about halfway to 

 the bottom of the body cavity. Such an arrangement 

 would be represented by a bottle with its neck turned 

 inward. In this suspended sac, which is somewhat con- 



FIG. 236. Dissected Actinia: a, the thick opaque skin 

 consisting of ectoderm, lined with muscular fibers; 

 c , the tubular tentacles communicating with the inter- 

 spaces, h, between the membranous vertical folds; 

 gi S ' > orifices in the walls allowing passage of respira- 

 tory water from one compartment to another ; d, mouth 

 leading to gastric cavity, e. 



