. Mark anniversaire volume ; à Edward Laurens Mark, Hersey Professeur d'anatomie et directeur de l ?Zoo Laboratoire logique à l'Université de Harvard, à l'occasion de vingt-cinq ans de travail pour la promotion de la zoo ?logie. Mark, E. L. (Edward Laurens), 1847-1946 ; zoologie ; anatomie, physiologie ; comparatifs ; l'embryologie. Fig. I.-spirale chemin suivi par Diu- rella tigris Muller quand nager librement. Il est à noter que le côté portant la dent et ridge est toujours vers l'extérieur de la spirale (2 et 4 sont censés se trouver dans le plan du papier, tandis que 1 et 3 ci-dessous est est abov

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. Mark anniversary volume; to Edward Laurens Mark, Hersey Professor of Anatomy and Director of the Zoo?logical Laboratory at Harvard University, in celebration of twenty-five years of successful work for the advancement of zoo?logy. Mark, E. L. (Edward Laurens), 1847-1946; Zoology; Anatomy, Comparative; Physiology; Embryology. Fig. I.—Spiral Path followed by Diu- rella tigris Muller when freely swim- ming. It will be noticed that the side bearing the tooth and ridge is always toward the outside of the spiral (2 and 4 are supposed to lie in the plane of the paper, while 1 is below and 3 is above this plane). If x-y is a plane surface to which the spiral path is tangent, it will be ob- served that the side bearing the tooth is the only one to touch the plane. Now, as we have already seen, many organisms which habitually make use of this method of progres- sion have a form which is adapted to it. In this way is to be explained the asymmetry of the Infusoria. In the Rattulidce we have a group of animals, fun- damentally bilateral, which are taking on the spiral, un- symmetrical form as an adaptation to their method of movement. If we examine in detail the behavior of one of the Rattulidae, taking for example Diurella tigris Muller (Fig. E, 1), we find that it swims through the water in a spiral, of such a course that its twisted body forms a segment of the spiral path (Fig. I). The animal re- volves to the right and swerves continuously towards its dorsodextral side as it progresses. The result is a path almost exactly that which would be formed if the animal were moving on the inside of a hollow cylinder and the dorsodextral spiral ridge ran in a groove on the inner surface of the cylinder, which fitted it pre- cisely and had the same curvature. The effect is the same as that produced by the spiral grooves on the inner surface of a rifle-barrel, giving the ball a rotary motion in the axis of flight. The result is, here as in the rifle-ball, to make the axis of moveme

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