80 PRIMITIVE ANIMALS 



nerves with single roots which are purely sensory in 

 function and supply the skin in the head region. All 

 the succeeding nerves have two roots, a dorsal and a 

 ventral, thus corresponding to the spinal nerves of 

 the Vertebrates. It is thus seen that the central 

 nervous system of the Lancelet is in a much simpler 

 condition than that of the lowest Vertebrates, the 

 Fishes. In them the front end of the central nervous 

 system is swollen to form a complicated brain from 

 which the first ten nerves arise as cranial nerves, 

 specialized in connection with the elaborate sense 

 organs present on the Vertebrate head. In the 

 Lancelet the sense organs consist of a very simple 

 eye and a small pit, doubtfully an olfactory organ, 

 both of which are directly applied to the substance 

 of the brain. The brain itself is simply the un- 

 differentiated front end of the nerve-cord, and the 

 only nerves corresponding to the cranial nerves of 

 the Vertebrates are the two most anterior pairs. 

 The alimentary canal is again simpler in arrange- 

 ment than that of the Vertebrate ; it is not coiled, 

 and it has only one digestive gland in connection 

 with it. The pharynx is pierced by very numerous 

 gill-slits (Fig. 18) enabling w r ater to be passed out 

 of it to the exterior, and in this respect it has a 

 schematic resemblance to a Fish, though the gills 

 and gill-bars do not resemble those of a fish in any 

 detail. 



