In each ganglion the cells of this ganglionic investment penetrate as a median septum into the cord. A fissure is next formed, dividing this septum into two; it is subsequently continued for the whole length of the cord. Arthropoda. In the Tracheata and the Crustacea the development of the ventral cord is in the main similar to that in the Chætopods, while that of the supraoesophageal ganglia is as a rule somewhat more complicated. No such clear evidence of an independent development of these two parts, as in the case of the Chætopods, has as yet been produced. The most primitive type of nervous system amongst the Tracheata is that of Peripatus, where it consists of large supraoesophageal ganglia, continuous with a pair of widely separated but large ventral cords united posteriorly above the anus. These cords have an investment of ganglion-cells for their whole length, and are imperfectly divided into ganglia corresponding in number with the feet. [FIG. 241. SECTION THROUGH THE TRUNK OF AN EMBRYO OF PERIPATUS. The embryo from which the section is taken was somewhat younger than that of fig. 242. _sp.m._ splanchnic mesoblast; _s.m._ somatic mesoblast; _mc._ median section of body cavity; _lc._ lateral section of body cavity; _v.n._ ventral nerve cord; _me._ mesenteron.] The ventral cords are formed as two separate epiblastic ridges (fig. 241, _v.n_), continued in front into a pair of thickenings of the procephalic lobes, which are at first independent of each other, and from which a large part of the supraoesophageal ganglia takes its origin. After the latter have become separated from the epiblast an invagination of the epiblast covering them grows into each lobe (fig. 242), and becoming constricted from the superficial epiblast, which remains as the epidermis, forms a not unimportant part of the permanent supraoesophageal ganglia. In the Arachnida the mode of development of the nervous system is essentially the same, and the reader will find a detailed account of it for Spiders in Vol. II. pp. 447-451. The ventral cords are here formed as independent and at first widely separated strands (fig. 243, _vn_), which for a long time remain far apart; they are subsequently divided into ganglia and become united by transverse commissures. The supraoesophageal ganglia are formed as two independent thickenings of the procephalic lobes (fig. 244), which eventually separate from the superficial skin. There is formed however in each of them a semicircular groove (fig. 244, _gr_) lined by the superficial epiblast, which becomes detached from the skin, and is involuted to form part of the ganglia. [FIG. 242. HEAD OF AN EMBRYO PERIPATUS. (From Moseley.) The figure shews the jaws (mandibles), and close to them epiblastic involutions, which grow into the supraoesophageal ganglia. The antennæ, oral cavity, and oral papillæ are also shewn.] [FIG. 243. TRANSVERSE SECTION THROUGH THE VENTRAL PLATE OF AGELENA LABYRINTHICA.