NER rO us S YSTEM. 



337 



pair of areas of punctiform material, the substance of which soon 

 differentiates itself into nerve-fibres. These areas, by uniting from 

 side to side, give rise to the transverse commissures, and also by a 

 linear coalescence to the longitudinal commissures of the cord. The 

 cellular parts of the band surrounding them become converted into a 

 ganglionic covering of the cord. 



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 develop- 

 ment of the ventral cord is in the main similar to that in the 

 Chaetopods, while that of the supracEsophageal 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 Chaitopods, has as yet 

 been produced. 



The most primitive type of 

 nervous system amongst the 

 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 invest- 

 ment of ganglion-cells for their 

 whole length, and are imper- 

 fectly divided into ganglia cor- 

 responding in number with the 

 feet. 



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 supra- 

 oesophageal 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 i. pp. 370 — 373. 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. 



B. E. II. 22 



Fig. 241. Section through the trunk 

 OF AN EMBRYO OF Periiatus. The emb'ryo 

 from which the section is taken was some- 

 what younger than that of fig. 242. 



sp.m. splanchnic mesoblast; s.m. somatic 

 mesoblast ; vie. median section of body cavity ; 

 Ic. lateral section of body cavity; v.n. ventral 

 nerve cord; me. mesenteron. 



