"It has been recognised by various writers, but notably by Gegenbaur and Haeckel, that a condition of radiate symmetry must have preceded the condition of bilateral symmetry in animal evolution. The Diblastula may be conceived to have been at first absolutely spherical with spherical symmetry. The establishment of a mouth led necessarily to the establishment of a structural axis passing through the mouth, around which axis the body was arranged with radial symmetry. This condition is more or less perfectly maintained by many Coelenterates, and is reassumed by degradation of higher forms (Echinoderms, some Cirrhipedes, some Tunicates). The next step is the differentiation of an upper and a lower surface in relation to the horizontal position, with mouth placed anteriorly, assumed by the organism in locomotion. With the differentiation of a superior and inferior surface, a right and a left side, complementary one to the other, are necessarily also differentiated. Thus the organism becomes bilaterally symmetrical. The Coelentera are not wanting in indications of this bilateral symmetry, but for all other higher groups of animals it is a fundamental character. Probably the development of a region in front of, and dorsal to the mouth, forming the _Prostomium_, was accomplished _pari passu_ with the development of bilateral symmetry. In the radially symmetrical Coelentera we find very commonly a series of lobes of the body-wall or tentacles produced _equally_--with radial symmetry, that is to say--all round the mouth, the mouth terminating the main axis of the body--that is to say, the organism being 'telostomiate.' The later fundamental form, common to all animals above the Coelentera, is attained by shifting what was the main axis of the body--so that it may be described now as the 'enteric' axis; whilst the new main axis, that parallel with the plane of progression, passes through the dorsal region of the body running obliquely in relation to the enteric axis. Only one lobe or outgrowth of those radially disposed in the telostomiate organisms now persists. This lobe lies dorsally to the mouth, and through it runs the new main axis. This lobe is the _Prostomium_, and all the organisms which thus develop a new main axis, oblique to the old main axis, may be called prostomiate." It will be seen from this quotation that the aboral part of the body is supposed to elongate to form the trunk, while the præoral region is derived from one of the tentacles. Before proceeding to further considerations as to the origin of the Bilateralia, suggested by the Pilidium type of larva, it is necessary to enter into a more detailed comparison between our larval forms. A very superficial consideration of the characters of these forms brings to light two important features in which they differ, viz.: (1) In the presence or absence of sense organs on the præoral lobe. [FIG. 229. TWO STAGES IN THE DEVELOPMENT OF TORNARIA. (After Metschnikoff.)