Outlines of comparative anatomy of vertebrates / by J.S. Kingsley.

  • John Sterling Kingsley
Date:
[1926]
Catalogue details

Licence: Public Domain Mark

Credit: Outlines of comparative anatomy of vertebrates / by J.S. Kingsley. Source: Wellcome Collection.

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    The Chordata are bilaterally symmetrical animals with meta- meric bodies, which agree in several features not found in the other groups. These are (i) a central nervous system, entirely on one side of the digestive tract; (2) the presence of gill slits in the young if not in the adult; (3) an unsegmented axial rod, the notochord, between the digestive tract and the nervous system. All of these features will be described later. There are three or four divisions of Chordata, the uncertainty depending upon the position to be accorded the Enteropneusta. These are worm-like animals, occurring in the sea and represented on our shores by Balanoglossus. What has been described as a notochord is a pocket from the digestive tract, lying in a curious proboscis above the mouth. The next division, the Tunicata, includes the (marine) ‘sea- squirts.’ They were long regarded as molluscs, but the discovery that the young have true gill slits, a nervous system on one side of the alimentary canal, and, above all, a notochord, placed them in the present association. Their young (larva?) are tadpole-like, the notochord is confined to the tail, but later the tadpole features are lost and with them the tail and notochord, and the adult is a sac-like animal with no resemblances to its former state, or to its allies. The third division, the Leptocardii, embraces Amphioxus and a few other marine, fish-like animals. They were long classed as fishes, but they are far more simple than any true fish. The body is markedly segmented, the gill slits are very numerous and the ex- cretory organs open separately to the exterior and are vermian in character. Stomach, vertebrae and heart are lacking and the brain and sense organs are very rudimentary, while jaws and paired appendages are absent. The last class, the Vertebrata, are most nearly related to the Leptocardii, but differ in many important respects. Thus there is always a skull and vertebral column; the brain is larger than the spinal cord; there are always nose, eyes and ears; a heart is present and the excretory organs open into a common duct on either side, with an external opening near the anus. Most of the characteristics of a vertebrate may be seen from the accompanying diagram. The body is bilaterally symmetrical, with anterior and posterior ends, dorsal and ventral sides well differen- tiated. There is no external segmentation, since the muscles are not directly attached to the skin, but a metameric arrangement of
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