Let us now leave this original structure of the mammal gut for a moment, in order to compare it with the alimentary canal of the lower Vertebrates, and of those Invertebrates that we have recognised as man's ancestors. We find, first of all, in the lowest Metazoa, the Gastraeads, that the gut remains permanently in the very simple form in which we find it transitorily in the palingenetic gastrula of the other animals; it is thus in the Gastremaria (Pemmatodiscus), the Physemaria (Prophysema), the simplest Sponges (Olynthus), the freshwater Polyps (Hydra), and the ascula-embryos of many other Coelenteria (Figures 2.233 to 2.238). Even in the simplest forms of the Platodes, the Rhabdocoela (Figure 2.240), the gut is still a simple straight tube, lined with the entoderm; but with the important difference that in this case its single opening, the primitive mouth (m), has formed a muscular gullet (sd) by invagination of the skin. (FIGURE 2.352. Gut of a human embryo, one-sixth of an inch long, magnified fifteen times. (From His. Showing: Epiglottis, Tongue, Hypophysis, Hepatic duct, Tail, Allantoic duct, Tail-gut, Umbilical cord, Larynx, Rudimentary lungs, Stomach, Pancreas, Bladder, Wolffian duct, Rudimentary kidneys.)) We have the same simple form in the gut of the lowest Vermalia (Gastrotricha, Figure 2.242, Nematodes, Sagitta, etc.). But in these a second important opening of the gut has been formed at the opposite end to the mouth, the anus (Figure 2.242 a). We see a great advance in the structure of the vermalian gut in the remarkable Balanoglossus (Figure 2.245), the sole survivor of the Enteropneust class. Here we have the first appearance of the division of the alimentary tube into two sections that characterises the Chordonia. The fore half, the head-gut (cephalogaster), becomes the organ of respiration (branchial gut, Figure 2.245 k); the hind half, the trunk-gut (truncogaster), alone acts as digestive organ (hepatic gut, d). The differentiation of these two parts of the gut in the Enteropneust is just the same as in all the Tunicates and Vertebrates.