2OO THE SPINAL NERVES OF AMPHIOXUS. 



It only remains for me to suppose that the specimens which 

 Stieda had at his disposal, were so shrunk as to render the 

 origin of the nerves very difficult to determine. 



The arrangement of the nerves of Amphioxus, according 

 to my own observations, is as follows. 



The anterior end of the central nervous system presents 

 on its left and dorsal side a small pointed projection, into 

 which is prolonged a diverticulum from the dilated anterior ven- 

 tricle of the brain. This may perhaps be called the olfactory 

 nerve, though clearly of a different character to the other nerves. 

 It was first accurately described by Langerhans 1 . 



Vertically below the olfactory nerve there arise two nerves, 

 which issue at the same level from the ventral side of the 

 anterior extremity of the central nervous system. These form 

 the first pair of nerves, and are the only pair which arise from 

 the ventral portion of the cerebro-spinal cord. The two nerves, 

 which form the second pair, arise also opposite each other 

 but from the dorsal side of the cord. The first and second 

 pair of nerves have both been accurately drawn and described 

 by Langerhans : they, together with the olfactory nerve, can 

 easily be seen in nervous systems which have been isolated by 

 maceration. 



In the case of the third pair of nerves, the nerve on the 

 right-hand side is situated not quite opposite but slightly be- 

 hind that on the left. The right nerve of the fourth pair is 

 situated still more behind the left, and, in the case of the 

 fifth pair, the nerve to the right is situated so far behind the 

 left nerve that it occupies a position half-way between the 

 left nerves of the fifth and sixth pairs. In all succeeding nerves 

 the same arrangement holds good, so that they exactly alternate 

 on two sides. 



Such is the arrangement carefully determined by me from 

 one specimen. It is possible that it may not be absolutely con- 

 stant, but the following general statement almost certainly 

 holds good. 



All the nerves of Amphioxus, except the first pair, have 

 their roots inserted in the dorsal part of the cord. In the case of 



