NERVOUS SYSTEM. 413 



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 segmenta- 

 tion of the cords subsequently becomes lost. The supracesophageal 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 supracesophageal ganglion is formed in this genus as an 

 unpaired band. 



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. (i) the supracesophageal 

 ganglia, and (2) the ventral cord. 



In the simpler forms, Chaetopoda and Gephyrea, the supra- 

 cesophageal ganglia are usually stated to be formed as an 

 unpaired thickening at the apex of the praeoral lobe, which in 

 most cases becomes subsequently bilobed. 



In the Arthropoda the unpaired praeoral lobe of the Chaeto- 

 poda is replaced by the so-called procephalic lobes, which are 

 themselves bilobed ; and the supracesophageal 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 continu- 

 ously 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. 



The form of nervous system of the Chastopoda, 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 supracesophageal 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). 



