i88 COMPARATIVE ANATOMY 



lined by an epithelium filled with brown pigment like the 

 atrial epithelium, and are clearly epiblastic involutions of 

 the atrial wall. Hence these brown -funnels, as they are 

 called, if they are really excretory which is not certain- 

 may properly be called nephridia. 



The central nervous system of Amphioxus presents many 

 interesting features (fig. 45, A). It is a simple thick-walled 

 tube lying dorsal to and nearly co-extensive in length with 

 the notochord. Posteriorly the nerve tube tapers down to 

 a point ; anteriorly it ends abruptly at the level of the first 

 myotome, some little distance behind the front end of the 

 notochord. The nerve cord is perforated throughout its 

 extent by a minute central canal, which is evidently the 

 persistent lower part of a cleft which extended from the 

 upper surface of the cord through about two-thirds of its 

 thickness, but the upper walls of the cleft have become 

 closely approximated, leaving only the minute canal below. 

 At the anterior end of the cord the canal widens out to 

 form an almost globular space known as the cerebral vesicle. 

 In young specimens this vesicle opens into the olfactory pit 

 by an aperture called the neuropore, but in later life the 

 pore closes up and the olfactory pit is only attached to the 

 roof of the cerebral vesicle by a solid cord of cells. Except 

 for this cerebral vesicle, and the specialisation of the two 

 anterior pairs of nerves, there is no trace of a brain. Nerves 

 are given off laterally to the myotomes and body-wall from 

 the whole length of the neural cord. With the exception of 

 the first two pairs, these nerves consist of a dorsal or sensory, 

 and a ventral or motor root, to this extent resembling the 

 spinal nerves of higher vertebrates, but their arrangement is 

 in many respects different and more primitive. In the first 

 place the nerves given off from opposite sides of the cord 

 are not symmetrical, but alternate with one another, this 

 disposition being obviously correlated with the asymmetry of 

 the myotomes. As is the case in higher vertebrates, the nerve 

 fibres of a dorsal root are gathered into a single nerve trunk, 

 while the ventral fibres emerge separately from the nerve cord ; 

 but in Amphioxus the dorsal and ventral roots are not given 

 off from the same section of the cord, the latter being placed 

 some way in front of the former. Again, there is no ganglion 

 on the dorsal or sensory root, and the two sets of nerve fibres, 



