advanced to a far higher condition. Passing to the jointed animals like worms and insects, we find nervous mechanisms that are still more intricate, and with their advance in structural respects there is a corresponding and correlated progress in their functions. Because the whole organism has developed more highly differentiated groups of organs to perform the several biological tasks, such as eating and respiring and moving, it is necessary for the nervous structures concerned with the direction of these actions to become more efficient. An earthworm avoids the light of day and digs its burrow and seeks its food by wonderfully cooerdinated activities of its muscles and other parts, which are controlled by a double chain of ganglia along its ventral side, connected with a similar pair of grouped nerve-cells above the anterior part of the digestive tract. The ganglia of each segment exercise immediate supervision over the structures of their respective territory, while they pass on impulses to other ganglia so that movements involving many segments can be properly adjusted. Everything an earthworm does is controlled by the cells grouped in these ganglia, or scattered along the intervening connecting cords. We speak of its acts as instinctive, employing a term which seems to indicate a different kind of operation carried on by the nervous system, but a moment's thought will show that an instinctive act is simply a complex group of reflex acts. The physical basis and ultimate unit is a cell, and the functional unit is likewise a cell act; therefore the seeming difference proves to be one merely of degree and not of kind. The greater complexity of the worm's nervous system as compared with that of _Hydra_ gives to the whole mechanism a plasticity that diverts the attention from the mechanical nature of the entire instinctive act and of its basic cell elements.