Page:Popular Science Monthly Volume 75.djvu/142

138 finally cover its tentacles and month by puckering in the oral edge of its column. In this contracted state it may remain hours at a time, and when it eventually expands it does so by relaxing its muscles and refilling its body with sea water. A beam of strong sunlight, if thrown upon an expanded Metridium several feet under water, will usually call forth the same contraction as mechanical stimulation does.

When the exterior of a Metridium is tested locally, its receptiveness for certain stimuli is found to be quite diverse. The animal makes no movements when dissolved food-substances are cautiously discharged upon the external surface of its  column, though this very area is sensitive to mechanical stimulation. Precisely the reverse is true of the lips; these organs are easily stimulated by dissolved food-products, but no reaction occurs even when they are punctured by a needle. Both mechanical and chemical stimulation, however, are effective on the tentacles and vigorous responses can be called forth from even distant parts of the body by the application of either of these forms of stimuli to the tentacles. Since these reactions, as just intimated, often involve responses in very different parts of the animal from those to which the stimulus is applied, it follows that we are dealing with a process justly regarded as nervous for transmission in this case is not accompanied with any observable motion. The surface of a sea-anemone may then be pictured as a true receptor surface partly differentiated in different regions for particular classes of stimuli, but not so far specialized that it can be described as made up of sense organs.

An examination of the structure of the ectoderm (Fig. 2) will do much to make clear the mechanism by which the reactions of sea-anemones are carried out. The ectoderm of these animals is a modified epithelium in which three definite layers can be distinguished. The outermost of these forms more than half the thickness of the total layer and is a true columnar epithelium. It contains, in addition to ordinary epithelial cells, gland-cells and nettle-cells, and, what is of more importance to us, sense-cells. These sense-cells are long, narrow bodies whose distal ends are armed with a sensory bristle which, under ordinary conditions, projects into the surrounding sea water and whose proximal ends run out into finely branched, nervous processes which intermingle with similar processes from other cells. The complex made by the interweaving of immense numbers of these processes constitutes the second layer of the ectoderm, the nervous layer, and this layer often contains in addition to the large amount of fibrillar material derived