412 GEPHYREA. 



what is known on the subject is given in Vol. n. pp. 521 2. It 

 appears that the ventral cord may either arise as an unpaired 

 thickening of the epiblast (Isopoda), marked however by a 

 shallow median furrow, or from two cords which eventually 

 coalesce 1 . It is not certain how far the supracesophageal 

 ganglia are usually in the first instance continuous with the 

 ventral cord. In Astacus, the early stages of which have been 

 elaborately investigated by Reichenbach (No. 331), they are 

 stated to be so ; the supracesophageal ganglia are moreover de- 

 scribed by this author as having a somewhat complicated origin. 

 Five elements enter into their composition. There is first 

 formed a pair of pits on the procephalic lobes, which become 

 very deep during the Nauplius stage, and are continuous with a 

 pair of epiblastic 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 cesophageal com- 

 missures ; 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 supracesophageal 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, corresponding with the segments. The development of 

 the commissural and ganglionic portions takes place much as in 

 the Chaetopoda. 



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 

 epiblast. 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 

 volume, there is a pair of ventral cords 2 . In correspondence with a general 

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



1 Reichenbach (No. 331) holds that the walls of the groove between the two 

 strands of the ventral cords become invaginated and assist in the formation of the 

 ventral cord. 



8 " Ueber Entwicklungsgeschichte d. Echiurus." Arbeit, a. d. zool. Instit. Wien 

 Vol. ill. 1880. 



