Page:A Text-book of Animal Physiology.djvu/55

 budding, and when cut into portions each may become a complete individual. However, under other circumstances, near the bases of the tentacles the body wall may protrude into little masses (testes), in which cells of peculiar formation (spermatozoa) arise, and are eventually set free and unite with a cell (ovum) formed in a similar protrusion of larger size (ovary). Here, then, is the first instance in which distinctly sexual reproduction has been met in our studies of the lower forms of life. This is substantially the same process in Hydra as in mammals. But, as both male and female cells are produced by the same individual, the sexes are united (hermaphroditism); each is at once male and female.

Any one watching the movements of a Polyp, and comparing it with those of a Bell-animalcule, will observe that the former are much less machine-like; have greater range; seem to be the result of a more deliberate choice; are better adapted to the environment, and calculated to achieve higher ends. In the absence of a nervous system it is not easy to explain how one part moves in harmony with another, except by that process which seems to be of such wide application in nature, adaptation from habitual simultaneous effects on a protoplasm capable of responding to stimuli. When one process of an Amœba is touched, it is likely to withdraw all. This we take to to [sic] be due to influences radiating through molecular movement to other parts; the same principle of action may be extended to Hydra. The oftener any molecular movement is repeated, the more it tends to become organized into regularity, to become fixed in its mode of action; and if we are not mistaken this is a fundamental law throughout the entire world of living things, if not of all things animate and inanimate alike. To this law we shall return.

But Hydra is a creature of but very limited specializations; there are neither organs of circulation, respiration, nor excretion,