NERVOUS SYSTEM. 409 



Tracheata is that of Peripatus, where it consists of large supra- 

 cesophageal 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. 



The ventral cords are formed as two separate epiblastic 

 ridges (fig. 241, vji), continued in front into a pair of thickenings 



S./ft 



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.HI, somatic mesoblast; inc. median section of body 

 cavity; lc. lateral section of body cavity; v.n. ventral nerve cord; me. mesenteron. 



of the procephalic lobes, which are at first independent of each 

 other, and from which a large part of the supracesophageal 

 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 supracesophageal 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 supracesophageal ganglia are formed as two independent 



