340 GEPHYBEA. 



during the Nauplius stage, and are continuous with a pair of epi- 

 blastic ridges which pass round the mouth, and join the ventral 

 cords just described. The walls of the pits are believed to form 

 a part of the embryonic ganglia which gives rise to the retina as 

 well as to the optic ganglia. The ridges form the remainder of the 

 ganglia and the oesophageal commissures ; while the fifth element is 

 supplied by a median invagination in front of the mouth, which 

 appears at a much later date than the other parts. 



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, cor- 

 responding with the segments. The development of the commissu- 

 ral and ganglionic portions takes place much as in the Chsetopoda. 



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 

 e[)iblast. 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 volvime, there is a pair of ventral cords'. In correspondence with a 

 general segmentation of the body, which is subsequently lost, these cords 

 become segmenterl. The two cords unite in the median line, and Hat^chek, 

 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 seg- 

 mentation 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 

 ISpengel in Bonellia, and the supraoesophageal 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. (1) the supraoesophageal ganglia, 

 and (2) the ventral cord. 



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

 geal ganglia are usually stated to be formed as an unpaired thickening 

 at the apex of the praeoral lobe, whi^h in most cases becomes subse- 

 quently bilobed. 



In the Arthropoda the unpaired prseoral lobe of the Cha;topoda is 

 replaced by the so-called procephalic lobes, which are themselves 

 bilobed ; and the supraoesophageal ganglia are formed of two in- 

 dependent 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 mdependent un- 



1 "Ueber Entwicklungsgeschichte d. Echiurus." Arbeit, a. d. zool. Instit. Wiev, 

 Vol. ni. 1880. 



