But before we go into the regular coelomation of the amphioxus, we will glance at that of the arrow-worm (Sagitta), a remarkable deep-sea worm that is interesting in many ways for comparative anatomy and ontogeny. On the one hand, the transparency of the body and the embryo, and, on the other hand, the typical simplicity of its embryonic development, make the sagitta a most instructive object in connection with various problems. The class of the chaetogatha, which is only represented by the cognate genera of Sagitta and Spadella, is in another respect also a most remarkable branch of the extensive vermalia stem. It was therefore very gratifying that Oscar Hertwig (1880) fully explained the anatomy, classification, and evolution of the chaetognatha in his careful monograph. The spherical blastula that arises from the impregnated ovum of the sagitta is converted by a folding at one pole into a typical archigastrula, entirely similar to that of the Monoxenia which I described (Chapter 1.8, Figure 1.29). This oval, uni-axial cup-larva (circular in section) becomes bilateral (or tri-axial) by the growth of a couple of coelom-pouches from the primitive gut (Figures 1.76 and 1.77). To the right and left a sac-shaped fold appears towards the top pole (where the permanent mouth, m, afterwards arises). The two sacs are at first separated by a couple of folds of the entoderm (Figure 1.76 pv), and are still connected with the primitive gut by wide apertures; they also communicate for a short time with the dorsal side (Figure 1.77 d). Soon, however, the coelom-pouches completely separate from each other and from the primitive gut; at the same time they enlarge so much that they close round the primitive gut (Figure 1.78). But in the middle line of the dorsal and ventral sides the pouches remain separated, their approaching walls joining here to form a thin vertical partition, the mesentery (dm and vm). Thus Sagitta has throughout life a double body-cavity (Figure 1.78 lk), and the gut is fastened to the body-wall both above and below by a mesentery--below by the ventral mesentery (vm), and above by the dorsal mesentery (dm). The inner layer of the two coelom-pouches (mv) attaches itself to the entoderm (ik), and forms with it the visceral wall. The outer layer (mp) attaches itself to the ectoderm (ak), and forms with it the outer body-wall. Thus we have in Sagitta a perfectly clear and simple illustration of the original coelomation of the enterocoela. This palingenetic fact is the more important, as the greater part of the two body-cavities in Sagitta changes afterwards into sexual glands--the fore or female part into a pair of ovaries, and the hind or male part into a pair of testicles.