Biology of the vertebrates : a comparative study of man and his animal allies / by Herbert Eugene Walter.

  • Walter, Herbert Eugene, 1867-1945.
Date:
1928
Catalogue details

Licence: In copyright

Credit: Biology of the vertebrates : a comparative study of man and his animal allies / by Herbert Eugene Walter. Source: Wellcome Collection.

    39/824 (page 7)
    V. VERTEBRATE CHARACTERISTICS In the vertebrate or craniate type (see page 15) there are certain distinguishing features that serve to mark off this type from all others. Of these the most diagnostic are the following: 1. A Living Endoskeleton The skeleton, primarily a dorsal axis or vertebral column to which other parts are attached, is inside the body. It is made up of living tissues that continue to grow and so to meet the demands for an adequate scaffolding in a body of increasing size. This is why it is possible for vertebrates to become the largest known animals. Among non-vertebrates, such as molluscs and arthro- pods, the skeleton is outside the body, forming an imprisoning armor. Since it is a secreted structure, and consequently dead, it does not conveniently allow for extended growth, so that no one ever heard of a clam, a lobster, or an insect as large as an elephant, a whale, or a dinosaur. 2. A Notochord Not every vertebrate has a backbone, made up of separate parts as the name “ vertebrate ” would indicate, but all possess, at least early in life, a dorsal axis of peculiar structure, called the notochord. A few primitive vertebrates retain the notochord throughout life but in most cases it is a transient, skeletal axis, which later is re- placed by the backbone. 3. A Single, Tubular, Dorsal Nerve Cord The central nervous system extends along the dorsal side of vertebrates in the form of a single tube, a result brought about by the manner of its development. This tube has a hollow en- largement at the end, called the brain. In very primitive animals, which have not yet acquired a definite nervous system, such as Hydra and other coelenterates, the outside of the body serves the purpose of sensory mediator. Any structure as delicate and spe- cialized as the nervous system, however, cannot become efficient as long as it remains on the outside. Hence, in all animals except the simplest, the nervous system becomes protectively buried within the body. In vertebrates the burial is by the process of invagination, that is, the outer layer of the body along the dorsal side grows so rapidly that a groove is formed, the walls of which