A. Ideal ancestral larval form. B. Larval form from which the Trochosphere larva may have been derived. C. Larval form from which the typical Echinoderm larva may have been derived. _m._ mouth; _an._ anus; _st._ stomach; _s.g._ supraoesophageal ganglion. The black lines represent the ciliated bands.] A circumoral nerve-ring, if longitudinally extended, might give rise to a pair of nerve-cords united _in front and behind_--exactly such a nervous system, in fact, as is present in many Nemertines[143] (the Enopla and Pelagonemertes), in Peripatus[144], and in primitive molluscan types (Chiton, Fissurella, etc.). From the lateral parts of this ring it would be easy to derive the ventral cord of the Chætopoda and Arthropoda. It is especially deserving of notice in connection with the nervous system of the above-mentioned Nemertines and Peripatus, that the commissure connecting the two nerve-cords behind is placed on the _dorsal_ side of the intestine. As is at once obvious, by referring to the diagram (fig. 231 B), this is the position this commissure ought, undoubtedly, to occupy if derived from part of a nerve-ring which originally followed more or less closely the ciliated edge of the body of the supposed radiate ancestor. [143] _Vide_ Hubrecht, "Zur Anat. und Phys. d. Nerven-System. d. Nemertinen," _Kön. Akad. Wiss._, Amsterdam; and "Researches on the Nervous System of Nemertines," _Quart. Journ. of Micr. Science_, 1880. [144] _Vide_ F. M. Balfour, "On some points in the Anat. of Peripatus capensis," _Quart. Journ. of Micr. Science_, Vol. XIX. 1879. The fact of this arrangement of the nervous system being found in so primitive a type as the Nemertines tends to establish the views for which I am arguing; the absence or imperfect development of the two longitudinal cords in Turbellarians may very probably be due to the posterior part of the nerve-ring having atrophied in this group. It is by no means certain that this arrangement of the nervous system in some Mollusca and in Peripatus is primitive, though it may be so. In the larvæ of the Turbellaria the development of sense organs in the præoral region is very clear (fig. 222 B); but this is by no means so obvious in the case of the true Pilidium. There is in Pilidium (fig. 232 A) a thickening of epiblast at the summit of the dorsal dome, which might seem, from the analogy of Mitraria, etc. (fig. 233), to correspond to the thickening of the præoral lobe, which gives rise to the supraoesophageal ganglion; but, as a matter of fact, this part of the larva does not apparently enter into the formation of the young Nemertine (fig. 232). The peculiar metamorphosis, which takes place in the development of the Nemertine out of the Pilidium[145], may, perhaps, eventually supply an explanation of this fact; but at present it remains as a still unsolved difficulty. [145] _Vide_ Vol. II. p. 204.