The form of nervous system of the Chætopoda, Arthropoda, and Gephyrea is clearly therefore to be derived, as was first pointed out by Gegenbaur, from a more or less similar type to that now found in the Nemertines; and as suggested in the chapter on larval forms (_vide_ p. 378) may perhaps be derived from the elongation of a circular ring, of which the anterior end has become developed into the supraoesophageal ganglia, the lateral parts into the two lateral strands, while the posterior part persists in some forms in the junction of the ventral cords above the anus (Enopla and Peripatus). Mollusca. While study of the anatomy of the nervous system of the Mollusca, especially of certain primitive genera (Chiton, Haliotis, Fissurella, &c.) leaves little doubt that it is formed on the same type as that of the groups just spoken of, the development, so far as our imperfect knowledge enables us to make definite statements on the subject, is somewhat abnormal[156]. [156] _Vide_ Vol. II., pp. 273, 274. In the Gasteropoda and Pteropoda the supraoesophageal ganglia appear most probably to be developed either as paired thickenings of the epiblast of the velar area, or as invaginated pits of the velar area, which become detached from the surface, and then become solid (Hyaleacea and Limax). In either case the supraoesophageal ganglia appear to be developed quite independently of the pedal ganglia. The latter, as might be anticipated, are earlier in their development and more constant than the various visceral ganglia; and, if the views above expressed are correct, are homologous with the ventral cord of the Chætopods and Arthropods. Their actual development is very imperfectly known. The most precise statements on the subject, viz. those of Bobretzky and Fol, would lead us to suppose that they arise in the mesoblast, but it seems more probable that they are formed as thickenings of the sides of the foot. In the Cephalopods all the ganglia are stated to be differentiated in the mesoblast (Lankester, Bobretzky). Hatschek[157] has recently given a detailed description of the development of the supraoesophageal and pedal ganglia of Teredo. He finds that the former ganglia arise as an unpaired thickening of the epiblast in the centre of the velar area, and the latter as an unpaired thickening of the epiblast of the ventral side of the body between the mouth and the anus. The two ganglia would thus seem to be disconnected with each other in their development. [157] "Ueber Entwicklungsgeschichte von Teredo." _Arbeit. a. d. zool. Instit. Wien_, Vol. III. 1880. (327) F. M. Balfour. "Notes on the development of the Araneina." _Quart. J. of Micr. Science_, Vol. XX. 1880. (328) B. Hatschek. "Beitr. z. Entwicklung d. Lepidopteren." _Jenaische Zeitschrift_, Vol. XI. 1877. (329) N. Kleinenberg. "The development of the Earthworm, Lumbricus Trapezoides." _Quart. J. of Micr. Science_, Vol. XIX. 1879.