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  Section: Anatomy of Vertebrate Animals » The Provinces of the Vertebrata - The Class Pisces
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The Ichthyopsida


I.-The Ichthyopsida
    1. Have the epidermic exoskeleton either absent, or very slightly represented.
    2. The spinal column may persist as a notochord with a membranous sheath, or it may exhibit various degrees of chondrification or ossification. When the vertebrae are distinct, their centra have no epiphyses.
    3. The skull may be incomplete and membranous, more or less cartilaginous, or osseous. When membrane bones are developed in connection with it, there is a large parasphenoid. The basisphenoid is always small, if it be not absent.
    4. The occipital condyle may be absent, or single, or double. When there are two occipital condyles they belong to the ex-occipital region, and the basi-occipital region is unossified or very imperfectly ossified.
    5. The mandible may be absent, or be represented only by cartilage. If membrane bones are developed in connection with it, there is usually more than one on each side. The articular element may be ossified or not, and may be connected with the skull by the intermediation of a quadrate and a hyomandibular element, or by a single fixed plate of carti lage representing both these and the pterygo-palatine arch. A stapes may be present or absent.
    6. The alimentary canal may or may not terminate in a cloaca. When there is no cloaca, the rectum opens in front of the urinary organs.
    7. The blood-corpuscles are always nucleated, and the heart mav be tubular, bilocular, or trilocular.
    8. There are never fewer than two aortic arches in the adult.
    9. Respiration takes place by branchiae during part, or the whole, of life.
    10. There is no thoracic diaphragm.
    11. The urinary organs are permanent Wolffian bodies.
    12. The cerebral hemispheres may be absent, and are never united by a corpus callosum.
    13. The embryo has no amnion, and, at most, a rudimentary allantois.
    14. There are no mammary glands.


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