58 



communication with the exterior. Possibly, as Sedgwick* has 

 suggested, the gill slits may, like the nephridia, be merely 

 specialised parts of the original coelom, which was probably 

 formed as a series of diverticula from the archenteron. The 

 nervous system in place of being concentrated in a pair of 

 cords, running more or less ventrally, as in all the higher 

 Vermes and most of the groups derived from them, appears in 

 the primitive Chordata to have moved dorsally (probably in 

 consequence of some peculiarity in the habits or mode of life 

 of the animal), so that eventually the two bands came to 

 occupy a position above the alimentary canal and in the 

 median dorsal line (compare fig. 13, E, and fig. 19). The 

 anterior portion of the primitive nervous system (as seen in 

 the vermean ancestor) which occupied the prae-oral lobe 

 remained as the foremost part of the Chordate nervous 

 system, which no longer crossed the alimentary canal (as it 

 does in the higher Vermes and Arthropoda) but lay entirely 

 upon its dorsal side. The chief modification in the structure 

 of the Proto-Chordata was the formation of a stiff axis, the 

 notochord, running longitudinally between the alimentary 

 canal and the dorsal nervous system. It was probably 

 formed of endoderm cells from the dorsal wall of the alimen- 

 tary canal, but what its primary function was, or what organ 

 in any existing group of Vermes corresponds to it is 

 unknown.! 



The lowest forms of the Chordata which we know are 

 the Urochorda or Tunicata, but they are certainly a 

 degenerate group which diverged from an ancestral Proto- 

 Chordate more or less resembling Ajipendicidaria, or better 

 still, the tailed larval Ascidian. In these forms the notochord 



* Quart. Jourv. Micros. Sc, vol. xxiv, No. xciii, p. 43. 



t Hubrecbt {Quart. Joiiru. Micros. Sc, vol. xxiii, July, 1883,) has 

 suggested that it was derived from the sheath of the proboscis of the 

 Nemerteaiis. 



