History of the human body / by Harris Hawthorne Wilder.

  • Harris Hawthorne Wilder
Date:
1909
Catalogue details

Licence: Public Domain Mark

Credit: History of the human body / by Harris Hawthorne Wilder. Source: Wellcome Collection.

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    as teeth. A most important distinction between Amphioxns and the cyclostomes, however, lies in the fact that the latter possess a definite head, with brain and sense organs, parts which exist only in a rudimentary or potential sense in Am- phioxus. In distinction from the cyclostomes, or “ round-mouths,” are the true vertebrates, which are termed gnathostomes, or “ jaw-mouths,” the possession of jaws being a constant char¬ acteristic of the entire group. The lowest class of gnathos¬ tomes is that of the fishes (Pisces), but these are in turn subdivided into several groups, some of which represent lateral branches, that is, specializations along definite direc¬ tions, and thus not in the direct line of human history. The most primitive group is that of selachians, which comprises the sharks and dog-fish, and the skates or rays. This group of animals is absolutely fundamental for the morphologist and represents the first great stage in the main line of verte¬ brate history. Selachians have a wholly cartilaginous skele¬ ton, the mouth upon the lower side of the head and not at the anterior end, as in other fish, and five gill-slits which open separately and free, not covered by an operculum (gill-flap). Their position in the tree is clearly in the main line above the cyclostomes. The ganoid fishes are also of great importance to us. They represent a few remnants of what was the dominant group during the Devonian epoch and are the direct de¬ scendants of the selachians. As in the case of all such rem¬ nants, they are extremely diverse in structure among them¬ selves and are placed in a single group rather more for con¬ venience than because of a very close relationship to one another. They are characterized by the tendency of the scales to fuse into bony plates, a tendency which in the past resulted in the development of a special group, the placo- derms, which were entirely covered by a suit of mail formed in this way. Similar plates cover the head in all modern ganoids and they occur in rows along the body in a few forms (sturgeons). The skeleton is mainly cartilaginous in