and respiratory.  The first function to have cells specially devoted to it is the reproductive; this is a function absolutely necessary for the maintenance of the species.  For the nutritive cells die when they have brought the reproductive cells to their full development.  These few nutritive cells represent the body of all higher animals in contrast with the reproductive elements.  And with the development of a body, death, as a normal process, enters the world.  The dominant function is here evidently the reproductive, and the whole body is subservient to this.

In hydra the union and differentiation of cells is carried further.  But the cells are still much alike and only slowly lose their own individuality in that of the whole animal.  This is shown in the fact that each entodermal cell digests its own particles of food, although the nutriment once digested diffuses to all parts of the body.  Also almost any part of the animal containing both ectoderm and entoderm can be cut off and will develop into a new animal.

But beside the reproductive cells and tissues hydra has developed a very simple digestive system, in which the newly caught food at least macerates and begins to be dissolved.  This is the second essential function.  The animal can, and the plant as a rule does, exist with only the lowest rudiments of anything like nervous or muscular power; but no species can exist without good powers of digestion and reproduction.  These essential organs must first develop and the higher must wait.  And the inner, digestive, layer of cells persists in our bodies as the lining of the mid-intestine.  We compared hydra therefore to a little patch of the lining of our intestine covered with a flake of epidermis; only these layers in hydra possess powers lost to the corresponding cells of our bodies in the process of differentiation.  Notice, please, that when cell or organ has once been developed it persists, as a rule, modified, but not lost.  Nature’s experiments are not in vain; her progress is very slow but sure.  But hydra has also the promise of better things, traces of muscular and nervous tissue.  There are still no compact muscles, like our own, much less ganglion or brain or nerve-centre of individuality.  The tissues are diffuse, but they are the materials out of which the organs of higher animals will crystallize, so to speak.  Notice also that these higher muscles and nerves are here entirely subservient to, and exist for, digestion and reproduction.

In the turbellaria the reproductive system has reached a very high grade of development.  It is a complex and beautifully constructed organ.  The digestive system has also vastly improved; it has its own muscular layers, and often some means of grasping food.  But it is slower in reaching its full development than the reproductive system.  But all the muscles are no longer attached to the stomach; they are beginning to assert their independence, and, in a rude way, to build a body-wall.  But they are in many layers, and run in almost all directions.  Some of these layers will disappear, but the most important ones, consisting of longitudinal and transverse fibres, will persist in higher forms.  Locomotion by means of these muscles is slowly coming into prominence.  They are no longer merely slaves of digestion.