POSITIONAL AND CIIORDOTONAL ORGANS. Posi tional organs as found in the simplest conditions are minute balls of mineral matter suspended in a flexible chib-shaped body, and are known as 'sense-clubs."1"hey are formed on the margin of the bell in many medusa,. In some eases they are by long hairs against which they strike When they move. In a still more ad vanced condition, they are situated in a depres sion, and the surrounding tissue grows over and about them, so as to inclose thein in a cavity. It is but a step (rain such a condition to the form of positional organs. called otocyst, which occurs in many invertebrates. An otocyst is a spherical cavity lined with cilia and containing one or more balls of mineral matter called otoliths. The otocyst is specially and fully innervated, the stimuli coming from the movements of the otolith against the cilia, accompanying changes in the position of the body. There is no evi dence to sluw that such organs are capable of detecting ',mild-waves. Otoeysts occur in
medusa, many kinds of worms, numerous mol lusks, a few echinoderms. and the higher erns ta•ea. In the latter, the vesicles often remain open and the otoliths are grains of sand; the vesicles are lined with delicate hairs, sonic of which are actually attached to the otoliths. In sects have no otoeysts, but in their place we find remarkable structures known as •cho•dotonar organs. which lie in the integument of various parts of the body. They are largest and most highly developed in grasshoppers, locusts, and crickets. and two or four of them are very large and are known as the 'tympana.' They lie on the sides of the abdomen or on the tibia' of the fore legs. Chordotonal organs seem to be par ticularly adapted for the detection of sound waves, but how they can serve as positional or gans is not clear.