. Studies in general physiology. Physiology; Phototropism; Geotropism; Reproduction; Irritability; Regeneration (Biology). 350 Studies in G-eneeal Physiology. body to come to rest upon its ventral surface. We cannot compel the animal to bring its back in contact with solid bodies, and at the same time expose its ventral surface to the water.' The question now arises whether these phenomena of orientation are a function of the brain as are the spontaneous progressive movements. Strange to say this is not the case. The brainless Thysano- zoon returns to the ventral position when it is laid upon
. Studies in general physiology. Physiology; Phototropism; Geotropism; Reproduction; Irritability; Regeneration (Biology). 350 Studies in G-eneeal Physiology. body to come to rest upon its ventral surface. We cannot compel the animal to bring its back in contact with solid bodies, and at the same time expose its ventral surface to the water.' The question now arises whether these phenomena of orientation are a function of the brain as are the spontaneous progressive movements. Strange to say this is not the case. The brainless Thysano- zoon returns to the ventral position when it is laid upon its back, only the reaction occurs more slowly than in the normal animal, or in that portion of the animal containing the brain. Reactions to light could not be demon- strated. 3. If, instead of making a complete transverse section of the animal, only the longitudinal nerves are cut, and the two pieces are left united with each other by a very thin bridge of protoplasm at one side (Fig. 100), the aboral piece is not innervated directly by the nerves from the brain. A conduction of the impulse by way of the lateral nerve plexus is, of course, still possible. When, immediately, after the operation, I laid such an animal upon the bottom of the aquarium the oralpieoe at once began to move, while the aboral piece tried to attach itself to the bottom. It responded, however, to the pull which the oral piece exerted upon it, and took part in a per- fectly co-ordinated manner in the progressive movements, as if no interruption had occurred. After some time the oral piece turned about, crept over the back of the aboral piece whereby the latter was dragged along passively, and laid FIG. 100 1 The righting of starfish which have been laid upon their backs is also only i case of stereotropism, and has nothing to do with the effects of gravity. Digitized by Microsoft®. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability
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