[154] Reichenbach (No. 331) holds that the walls of the groove between the two strands of the ventral cords become invaginated and assist in the formation of the ventral cord. In the Isopoda supraoesophageal ganglia are stated to arise as thickenings of the procephalic lobes, which become eventually detached from the epidermis. The ventral cord is at first unsegmented, but soon becomes partially divided by a series of constrictions into a number of ganglia, corresponding with the segments. The development of the commissural and ganglionic portions takes place much as in the Chætopoda. The Gephyrea approach closely the types so far dealt with, but the ventral cord in the Inermia is formed as an unpaired thickening of the epiblast. In Echiurus, as has been shewn by Hatschek in an interesting paper on the larva of this species, published since the appearance of the first volume, there is a pair of ventral cords[155]. In correspondence with a general segmentation of the body, which is subsequently lost, these cords become segmented. The two cords unite in the median line, and Hatschek, in accordance with his general view on this subject, states that their junction is effected by means of a median cord of invaginated epiblast. The segmentation of the cords subsequently becomes lost. The supraoesophageal ganglia arise as an unpaired median thickening of the procephalic lobe. No traces of segmentation in the ventral cord have been observed by Spengel in Bonellia, and the supraoesophageal ganglion is formed in this genus as an unpaired band. [155] "Ueber Entwicklungsgeschichte d. Echiurus." _Arbeit. a. d. zool. Instit. Wien_, Vol. III. 1880. In all the groups above considered the nervous system clearly presents the same type of development with various modifications. It is formed of two parts, viz. (1) the supraoesophageal ganglia, and (2) the ventral cord. In the simpler forms, Chætopoda and Gephyrea, the supraoesophageal ganglia are usually stated to be formed as an unpaired thickening at the apex of the præoral lobe, which in most cases becomes subsequently bilobed. In the Arthropoda the unpaired præoral lobe of the Chætopoda is replaced by the so-called procephalic lobes, which are themselves bilobed; and the supraoesophageal ganglia are formed of two independent halves; further complications in development are also generally found. There is not as yet sufficient evidence to decide whether the supraoesophageal ganglia were primitively developed continuously with, or independently of, the ventral cords. The ventral cord appears in the embryo as _two independent unsegmented strands_, although in a few cases (some Crustacea and Gephyrea) these cords, by an abbreviation in development, arise as an unpaired median thickening of the epiblast.