. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Running along the entire dorsal side of the body, then around the tail end and forward on the ventral side is a continuous median fin. This fold of the skin, as it passes around the tail end, expands to form a conspicuous caudal fin. As the fold reaches the re- gion of the atrial pore, anterior to the anal opening, it divides like a letter Y, and extends forward in ventro-lateral folds on either side of the body. It is out of persisting parts of similar fo
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Running along the entire dorsal side of the body, then around the tail end and forward on the ventral side is a continuous median fin. This fold of the skin, as it passes around the tail end, expands to form a conspicuous caudal fin. As the fold reaches the re- gion of the atrial pore, anterior to the anal opening, it divides like a letter Y, and extends forward in ventro-lateral folds on either side of the body. It is out of persisting parts of similar folds, which are laid down temporarily in the embryos of fishes, that fins are formed (Fig. 14). Of all known lower chordates, am- phioxus shows the best development of diagnostic chordate features. Extending the entire length of the body just dorsal to the digestive tract is a notochord (Fig. 15). Above the latter is the dorsal, hollow central nervous system which, instead of enlarging anteriorly into a brain, actually is more slender near its anterior end than throughout most of its length. Not far from the mouth, the digestive tract becomes greatly en- Fig. 14. Diagrams of the phylogene- tic development of unpaired and paired fins according to the fin-fold theory. a, primitive stage, with continuous fins; B, differentiated stage, with fins remain- ing after partial absorption of the primitive continuous fins. (After Wie- dersheim.) Myotomes «- Ventral , Nerve Root. Dorsal Fin Ray .â¢Neurocoele !}ââ¢-â Dorsal Nerve Root ^^â¢- Nerve Cord Notochord Myocommata â¢g<£fDorsal Aorta l^C"EpipharyngealGroove Dorso-Lateral Coelom ~râAtrial Cavity g^^m-Liver "hHL-- Ovary SS^râCardinal Vein ?A^âTransverse Muscle ^-liKp&l-Branchial Bars le Coelom âVentral Aorta Metapleural Fold Fig. 15. Cross section of amphioxus through posterior part of Please note that these images are extracted from scanned page images that may have been digitally enha
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Keywords: ., bookcentury1900, booksubjectanatomycomparative, booksubjectverte
