CRANIAL GANGLIA IN AMPHIOXUS. 125 



They are lateral to the ventral parts of the canal. They retain 

 primitive characters in that the cell body is adjacent to the canal 

 and that there is a single large dendrite which extends to the 

 periphery of the cord. The origin of the neurite from the den- 

 drite at some distance from the cell body is perhaps also a primi- 

 tive character so far as the vertebrate nervous system is con- 

 cerned. With respect to the disposition of these cells along the 

 cord it is evident that they form a more or less complete column 

 and that the neurites often run for some distance in the lateral 

 tracts to reach their roots (Fig. 7). 



As our knowledge of the nervous system cf Amphioxus in- 

 creases its effect is to oppose the tendency of recent years to min- 

 imize the relationship between AmpJiioxus and vertebrates, to 

 consider AinpJiioxns as far removed from typical vertebrates and 

 more closely related to invertebrates rather low in the scale. 

 While there is great significance in the similarity of the neph in- 

 dium (10) and eye -spots (11) of Amphioxus to those of some 

 worms, the close relation of the nervous system of Amphioxus 

 to that of vertebrates has also an unquestionable significance. 

 Amphioxus, indeed, contributes more toward bridging over the 

 gap between vertebrates and invertebrates than has usually been 

 supposed. 



The nervous system of Amphioxus agrees with that of lower 

 fishes in the following respects : 



(a) It is dorsal, hollow, and has separate dorsal and ventral 

 roots of definite composition. The canal has an enlargement at 

 the anterior end, the brain ventricle. 



() The dorsal roots consist of general cutaneous, visceral 

 sensory and visceral motor components. They contain also in 

 the head region fibers of special sense organs (olfactory or 

 gustatory ?). 



(c) Both kinds of sensory fibers have ganglion cells which are 

 situated either within the cord or in the root of the nerve in 

 essentially the same position as the spinal ganglia of vertebrates. 



(d) The two kinds of sensory fibers on entering the cord form 

 dorsal tracts similar to those in vertebrates. Many cutaneous 

 fibers show the bifurcation characteristic of these fibers in verte- 

 brates. 



