. Comparative zoology, structural and systematic : for use in schools and colleges. Zoology; Anatomy, Comparative; Physiology, Comparative. 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 excr
. Comparative zoology, structural and systematic : for use in schools and colleges. Zoology; Anatomy, Comparative; Physiology, Comparative. 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 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, Please note that these images are extracted from scanned page images that may have b
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1883
