1. The following factors play a part in the coagulation of the blood of arthropods : (a) Agglutination of blood cells. (b) The formation of a gelatinous mass, and secondarily the formation of fibrils from protoplasm exuded from cells and from protoplasm of cells entirely dissolved. (c) The spreading out of the blood cells during the next few hours after shedding of the blood. (d) The coagulation of a fibrinogenous substance. 2. The agglomeration of the blood cells leads to the formation of tissue-like structures. The formation of the fibrillar intercellular substance corresponds to the formation of fibrillar connective tissue. By changing the cytolytic medium in which the cells are suspended, the formation of an intercellular fibrillar substance can be prevented. By agglutination of cells which remain well preserved, an epithelial arrangement of the blood cells can be brought about. 3. Inside of the normal body the blood cells do not change their form ; this however takes place outside of the body, without the presence of certain specifically directing chemotropic stimuli. The cell protoplasm moves in contact with the solid surface to which it sticks. In a similar way the leucocytes react during an inflammatory process and a certain analogy also exists between this process and the movement of the epithelium during regeneration. 4. Coagulation of the blood can be prevented or inhibited by receiving it in different solutions, such solutions usually having also a preservative influence upon the cells. Oxalates act in a similar way to other salt solutions, that is if they are very concentrated. This points to the fact that in all probability the precipitation of calcium is, under these conditions, not the essential factor. Sodium and potassium sulphate have, in a similar way as sodium nitrate and sodium and potassium chloride, an inhibiting, influence upon coagulation. The blood cells themselves, however, are influenced in a different way by the two sulphates and the other salts. 5. The coagulation of the blood is inhibited by receiving the blood in gelatine solutions ; collecting it in oil is without influence. 6. The second coagulation of the lobster blood is inhibited by means different from those acting on the first coagulation. The facts observed during the second coagulation can be explained through the hypothesis of the action of a fibrin ferment present in the blood cells and in the muscle of the lobster, which however is not present in the blood cells or in the muscle of certain vertebrates. This points to a multiplicity of fibrin ferments in different species of animals and points to the identity of the ferment in different tissues of the same species of animals. This specificity in the one case and the want of it in the other case corresponds to a similar specificity and want of specificity as has been found to exist with the precipitins obtained through injection of certain body fluids or albuminous substances into different species of animals. 7. Potassium cyanide, urea and solutions of peptone inhibit the second coagulation. If these substances are add to the serum their inhibiting influence is in a reverse proportion to thire strength if they act directly upon the fibrin which contains the ferment. Potassium cyanide has the strongest, peptone the weakest action upon the fibrin, and conversely, peptone has the strongest and potassium cyanide the weakest action on being mixed with serum. 8. The necessity of the presence of calcium for the second coagulation can be easily proved. Ammonium chloride has a stronger inhibiting action on the second coagulation than sodium chloride. 9. Mechanical agencies, namely, pressure and traction, change the cell protoplasm into a system of fibers which cannot be distinguished in its physical properties and appearances from the extra-cellular fibers. The cell granules disappear under the influence of traction and pressure, just as the granules disappear spontaneously in the cells during coagulation of the blood. By traction and pressure exerted upon the blood serum free from cells, similar fibrillar structures can be produced as in the protoplasm of the cell. The fibrillar structure of the protoplasm seems therefore to be a secondary condition, produced by mechanical influence upon albuminous substances which may be common to the protoplasm of the cell and to solutions of certain albuminous substances. 10. By pressure and traction the cell protoplasm can be changed into fibers whose arrangement is determined by the direction in which the mechanical forces act; mechanical factors may also determine the direction of the fibrils in connective tissue and bone. 11. The significance of the blood corpuscles for the coagulation of the blood consists therefore in the following: (i) The cells themselves are transformed into substances similar to fibrin, (ii) They accelerate a coagulation taking place in the surrounding fluid; some facts point to the possibility that the coagulating substance contained in the serum has its origin in the blood cells. 12. Towards foreign substances introduced into the body of the animal, the blood corpuscles of Limulus do not behave as actively as the blood corpuscles of mammals. An active penetration into these foreign substances cannot be observed in Limulus, the cells only take part in the formation of the coagulum around the foreign substance.