Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 ii6 ELEMENTS OF BIOLOGY the zoologist because it contains in rudimentary form many of the characters of the higher animals. The type form usually given is Amphioxus. Sub-phylum IV. \^ertebrata (Fig. 65). Members of this phylum have a notochord during their early existence, but in later life it is VERTEBRATA PLEURAL CAVITy SPINAL COfID HINDBRAIN MIDBRAIN FORiBRAIN ORAL CAVITY EXCRETORY ORGAN BLADDER THACHEA HEART VISCERAL curr PERICARDIAL CAVI
Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 ii6 ELEMENTS OF BIOLOGY the zoologist because it contains in rudimentary form many of the characters of the higher animals. The type form usually given is Amphioxus. Sub-phylum IV. \^ertebrata (Fig. 65). Members of this phylum have a notochord during their early existence, but in later life it is VERTEBRATA PLEURAL CAVITy SPINAL COfID HINDBRAIN MIDBRAIN FORiBRAIN ORAL CAVITY EXCRETORY ORGAN BLADDER THACHEA HEART VISCERAL curr PERICARDIAL CAVITY •REPRODUCTIVE DUCT VERTEBRAE FIG. 65. A SCHEME OF VERTEBRATE ORGANIZATION Fig. 65.—Scheme of organization of Vertebrata. When lungs are present, as in Amphibia, Reptilia, Aves, and Mammalia, they communicate with the floor of the anterior region of the digestive canal by way of the trachea. Water breathers expel water through the visceral or branchial clefts, indicated in the figure. In mammals the lung cavities are separated from the pericardial cavity and the peritoneal region of the coelom. These partitions are indicated by horizontal and perpendicular planes. more or less completely replaced by a series of spool-like bones or cartilages forming what is known as the vertebral column. True skin composed of two layers, bony teeth, bone, hair, and feathers, are found only in the Vertebrata, although, of course, not all Ver- tebrata have all these characters. All Vertebrata exhibit segmenta- tion, have red blood, are bilaterally symmetrical, have two pairs of
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