. A manual of elementary zoology . Zoology. 384 MANUAL OF ELEMENTARY ZOOLOGY and an inferior vena cava, but the Dipnoi have what are at least very passable attempts at both of these. Like the fishes, however, Amphibia have only ten cranial nerves, their eggs are without shells, they lack the embryonic membranes known as the amnion and allantois (p. 506), and they start life as gilled larvae. They have usually no exoskeleton. Besides the sturdy, long-legged, tailless animals, known as frogs and toads {Anura), the long-bodied, short-legged, tailed Newts, or Urodela, belong to this group, and it
. A manual of elementary zoology . Zoology. 384 MANUAL OF ELEMENTARY ZOOLOGY and an inferior vena cava, but the Dipnoi have what are at least very passable attempts at both of these. Like the fishes, however, Amphibia have only ten cranial nerves, their eggs are without shells, they lack the embryonic membranes known as the amnion and allantois (p. 506), and they start life as gilled larvae. They have usually no exoskeleton. Besides the sturdy, long-legged, tailless animals, known as frogs and toads {Anura), the long-bodied, short-legged, tailed Newts, or Urodela, belong to this group, and it also comprises the Gymnophiona—small blind, limbless, and tail- less creatures, which live like earthworms in the soil of warm countries. The Gymnophiona have rings of small scales embedded in the skin, recalling those of the Teleostei, but probably really the last remains of a bony armour. Fig. 277.—Ccecilia, one of the Gymnophiona. an, Anus, in an enlarged view of the underside of the hinder end. Note the absence of taii. which existed in certain extinct newt-like Amphibians (Stegocephalt) and may perhaps be compared to the scales of ganoids. Upon the head of the Stegocephali some of the bony plates became a part of the skull, which was in other respects more highly developed than that of the frog. The skulls of Amphibia show a progressive simplification in Gymnophiona, newts, and frogs. The reader will find on pp. 499-502 of this book an account of the fish-like arrangement of the arteries of a tadpole, and their relation to that of a frog. Here it may be added that in various newts more than one pair of aortic arches persists in the adult. In the Common Newt {Molge) the ductus arteriosus remains well developed, giving an addi- tional pair of arches (the pulmonary); in the Salamander the missing third branchial arch, lying between the two. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - colorati
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1920
