In the flat worms, where a nervous system with a rudimentary brain is found, the reaction to chemical stimulation is not clear. This organism has specialized responses, among which is a movement toward food placed near it. But whether this is a reaction to chemical stimuli alone or combined with mechanical is not known. No taste organs have been found. Pits or depressions found on the lateral surface of the anterior end of the worm, and supplied with nerves from the brain, have been regarded as olfactory rather than as taste organs. In the annelid group, of which the earthworm may be taken as an example, there are well-defined chemical reactions, which more nearly resemble taste reactions than the cases previously mentioned. Here a positive reaction to food substances seems to occur only when these substances come into contact with the body. For instance, the characteristic burrowing reactions of the earthworm are not aroused by placing filter paper soaked in manure near them, but only when the paper is actually in contact with the body. Negative reactions, however, to strong chemical stimulation may take place without contact. Attempts have been made by Parker and Metcalf to show specialized taste reactions to different chemical substances by measuring the latent time in the responses to various substances brought into contact with the body. From such evidence as this it would appear that earthworms have specialized reactions to the chlorides of sodium, potassium, lithium, and ammonium, which are indistinguishable to the human taste sense, with their common salt taste. These results are interpreted as indicating qualitatively different effects of the stimuli. In these organisms it has been possible to discover taste organs, distinct from the olfactory organs. They are described as cup-shaped organs, which may be either depressions or prominences. They occur in large numbers and are widely scattered over the body. They are said, however, to be especially numerous at the edges of the mouth and within the mouth cavity. The crustacea, among which are the crabs and the lobsters, characterized by their hard shell-like covering, show certain specific reactions to chemical substances when these come into contact with the parts of the body near the mouth. Reactions to chemical stimuli applied to any part of the body of the crayfish have been reported by Bell. The positive reactions were such as to bring the substance toward the mouth and the negative reactions such as to remove the substance. Responses to such substances at a distance are uncertain. But it is difficult to differentiate between possible smell and taste reactions. The sense organs in these organisms are usually located upon the antennæ, or feelers, in the neighborhood of the mouth. Here there is a different kind of response to chemical and mechanical stimulation. No structures with a specific taste function have been described, although smell and tactile organs have been localized.