It is interesting in this connection to again note the effect of these salts on the secretion of urine. It is well known that practically all of them are diuretics, when introduced with a considerable amount of fluid. And even when the flow of urine has been greatly increased by the injection of m/6 NaCl solution, it can be still further augmented by the addition of, _e.g._, sodium citrate to the injection fluid. These salts constitute the well known class of saline diuretics. All salts do not, however, belong to this class, as is often stated. Calcium chloride, magnesium chloride, and to some extent strontium chloride exert exactly the opposite effect, inhibiting the action of the diuretics and diminishing the flow of urine. These salts might be termed antidiuretics. There is thus an entire analogy between the action of the saline diuretics on the kidney and that of the saline purgatives on the intestine, and also the action of calcium and magnesium is the same in both cases. And the analogy can be traced farther back to the production and inhibition of muscular twitchings in voluntary muscles, which was demonstrated by Loeb. The actual mechanism of the secretion of fluid into the intestine is difficult to determine. It seems improbable that a change in blood pressure plays any very important rôle, if indeed it has an influence at all. There is much evidence to show that many glands consisting of cells resembling those of the intestine roughly, secrete their characteristic fluids quite independently of blood pressure. In _Sida crystallina_, a small fresh-water crustacean, it was found[89] that if a small quantity of one of the saline purgatives or of BaCl_{2} or pilocarpine be added to the water in which these crustaceans are lying, there is not only a rapid increase of intestinal movement and a rapid evacuation of faeces, but there is also an increased secretion of fluid into the intestine, so that the whole lumen becomes filled with a pale greenish fluid. It was pointed out further that in this organism there is no closed blood vascular system, the blood simply running in wide channels in more or less definite directions. There can therefore exist nothing here comparable with the blood pressure of higher animals, and yet secretion normally takes place without changes in blood pressure. Further, it can be greatly increased by chemicals without an increase in blood pressure being possible. A similar secretion without blood pressure as a causative factor is seen in the skin of the common slug (Ariolimax). Here the secretion of the skin may be markedly increased by the injection or local application of a solution of any of the saline purgatives. This takes place equally well when the heart of the animal is removed, and also in an isolated portion of the animal, or in a piece of the skin cut off with the scissors. In these latter cases there can be no possibility of blood pressure taking a part in the secretion.