The ontogenetic facts that we gather from this sole survivor of the Acrania are the more valuable for phylogenetic purposes, as paleontology, unfortunately, throws no light whatever on the origin of the Vertebrates. Their invertebrate ancestors were soft organisms without skeleton, and thus incapable of fossilisation, as is still the case with the lowest vertebrates--the Acrania and Cyclostoma. The same applies to the greater part of the Vermalia or worm-like animals, the various classes and orders of which differ so much in structure. The isolated groups of this rich stem are living branches of a huge tree, the greater part of which has long been dead, and we have no fossil evidence as to its earlier form. Nevertheless, some of the surviving groups are very instructive, and give us clear indications of the way in which the Chordonia were developed from the Vermalia, and these from the Coelenteria. While we seek the most important of these palingenetic forms among the groups of Coelenteria and Vermalia, it is understood that not a single one of them must be regarded as an unchanged, or even little changed, copy of the extinct stem-form. One group has retained one feature, another a different feature, of the original organisation, and other organs have been further developed and characteristically modified. Hence here, more than in any other part of our genealogical tree, we have to keep before our mind the FULL PICTURE of development, and separate the unessential secondary phenomena from the essential and primary. It will be useful first to point out the chief advances in organisation by which the simple Gastraea gradually became the more developed Chordaea. We find our first solid datum in the gastrula of the Amphioxus (Figure 1.38). Its bilateral and tri-axial type indicates that the Gastraeads--the common ancestors of all the Metazoa--divided at an early stage into two divergent groups. The uni-axial Gastraea became sessile, and gave rise to two stems, the Sponges and the Cnidaria (the latter all reducible to simple polyps like the hydra). But the tri-axial Gastraea assumed a certain pose or direction of the body on account of its swimming or creeping movement, and in order to sustain this it was a great advantage to share the burden equally between the two halves of the body (right and left). Thus arose the typical bilateral form, which has three axes. The same bilateral type is found in all our artificial means of locomotion--carts, ships, etc.; it is by far the best for the movement of the body in a certain direction and steady position. Hence natural selection early developed this bilateral type in a section of the Gastraeads, and thus produced the stem-forms of all the bilateral animals.