. Principles of economic zoo?logy. Zoology, Economic. 190 BRANCH CHORDATA chord is a smooth, elastic rod typically developed from the endo- derm, extending along the median line between the alimentary tube and the central nervous system. It is encased in a tough sheath or membraneand "forms an elastic supporting ;. ih ^s X mr hij K h-. ^5 . i. ^ L d e h n af oh Fig. 152.—Ideal primitive vertebrate, seen from the left side: na, Nose; au, eye; md, mouth; (j, ear; , gill openings; x, notochord; mr, spinal tube; kg, gill-vessels; A', gill-intestine; hz, heart; ?, muscles
. Principles of economic zoo?logy. Zoology, Economic. 190 BRANCH CHORDATA chord is a smooth, elastic rod typically developed from the endo- derm, extending along the median line between the alimentary tube and the central nervous system. It is encased in a tough sheath or membraneand "forms an elastic supporting ;. ih ^s X mr hij K h-. ^5 . i. ^ L d e h n af oh Fig. 152.—Ideal primitive vertebrate, seen from the left side: na, Nose; au, eye; md, mouth; (j, ear; , gill openings; x, notochord; mr, spinal tube; kg, gill-vessels; A', gill-intestine; hz, heart; ?, muscles; ma, stomach; V, intestinal vein; c, body cavity; a, aorta; \, liver; d, small intestine; e, ovary; h, testes; », kidney-canal; a/, anus; \h, true or leather skin; oh, outer skin (epidermis); /, skin-fold, acting as a fin. (After Hackel.) In the higher forms the notochord is replaced by a segmented cartilaginous or bony vertebral column. These three characteristics may not be easily recognized by the beginner as he looks at the worm-like BaVanoglos'sus, the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Daugherty, Lewis Sylvester, 1857-; Daugherty, L. S. , Mrs. , 1859-. Philadelphia, London, W. B. Saunders
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