The Whence and the Whither of Man. A brief history of his origin and development through conformity to environment. Being the Morse Lectures of 1895 / by John M. Tyler.

Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: The Whence and the Whither of Man. A brief history of his origin and development through conformity to environment. Being the Morse Lectures of 1895 / by John M. Tyler. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    processes which urge the animal forward like a myriad of microscopic oars. In our bodies they are sometimes used to keep up a current, e.g., to remove foreign par- ticles from the lungs. The turbellaria is still covered with cilia, probably an inheritance from the gastrsea; for, while in smaller forms they may still be the prin- cipal means of locomotion, in larger ones the muscles are beginning to assume this function and the animal moves by writhing. The bilateral symmetry has arisen in connection with this mode of locomotion and is thus a mark of important progress. In the turbellaria we find for the first time a true body-wall distinct from underlying organs. The outer layer of this is a ciliated epithelium or layer of cells. Under this an elastic membrane may occur. Then come true body muscles, running transversely, longi- tudinally and dorso-ventrally. Between the external transverse and the internal longitudinal layers we often find two muscular layers whose fibres run diagonally. The body is well provided with muscles, but their ar- rangement is still far from economical or effective. Within the body-wall is the parencliym. This is a spongy mass of connectile tissue in which the other organs are embedded. The mouth lies in the middle, or near the front of the ventral surface. The intestine varies in form, but is provided with its own layers of longitudinal and transverse muscles, and usually has paired pouches extending out from it into the body parenchym. These seem to distribute the dissolved nutriment; hence the whole cavity is still often called a gastro-vascular cavity as serving both digestion and circulation. There is no anal opening, but indigestible material is still cast out through the mouth.