The _gastraeads_ (or animals with a primitive gut) form a small group of the lowest coelenterates, which is of great importance as the common ancestral group of all the metazoa. The body of these little swimming animals looks like a tiny (generally oval) vesicle, which has a simple cavity with one opening--the primitive gut and the primitive mouth. The wall of the digestive cavity is formed of two simple layers of cells, or epithelium, the inner of which--the gut-layer--is responsible for the vegetal activity of nourishment, while the outer, or skin-layer, discharges the animal functions of movement and sensation. The homogeneous sensitive cells of the skin-layer bear long, slender hairs or lashes (_cilia_), by the vibration of which the swimming motion is effected. The few surviving forms of gastræads, the gastræmaria (_trichoplacidae_) and cyemaria (_orthonectidae_), are extremely interesting, from the fact that they remain throughout life at a stage of structure which is passed by all the other metazoa (from the sponge to man) at the commencement of their embryonic development. As I have shown in my _Theory of the Gastraea_ (1872), a very characteristic embryonic form, the _gastrula_, is immediately developed from the _blastula_ in all the tissue animals. The germinal membrane (blastoderm), which represents the wall of the hollow vesicle, forms a depression at one side, and this soon sinks in so deep that the inner cavity of the vesicle disappears. The half of the membrane which bends in is thus laid on, and inside, the other half; the latter forms the _skin-layer_, or outer germinal layer (ectoderm or epiblast), and the former becomes the _gut-layer_, or inner germinal layer (endoderm or hypoblast). The new cavity of the cup-shaped body is the digestive stomach cavity (the _progaste_), and its opening is the primitive mouth (or _prostoma_).[19] The skin-layer, or ectoderm, is the primitive psychic organ in the metazoa; from it, in all the nerve animals, not only the external skin and the organs of sense, but also the nervous system, are developed. In the gastræads, which have no nerves, all the cells which compose the simple epithelium of the ectoderm are equally organs of sensation and of movement; we have here the tissue-soul in its simplest form. The platodaria, the earliest and simplest form of the platodes, seem to be of the same primitive construction. Some of these cryptocoela--the _convoluta_, etc.--have no specific nervous system, while their nearest relatives, the turbellaria, have already differentiated one, and even developed a vertical brain.